A CONTRIBUTION  TO  THE  CHEMISTRY  OF  THE 
TELLURATES 


A 


BY 


EDGAR  BURTON  HUTCHINS,  Jr. 


A THESIS  SUBMITTED  FOR  THE  DEGREE  OF  DOCTOR  OF  PHILOSOPHY 
UNIVERSITY  OF  WISCONSIN, 

1905 


Reprinted  from  the  Bulletin  of  the  University  of  Wisconsin 
Science  Series,“Vol.  3,  No.  3 


MADISON,  WISCONSIN 
July,  1905 


L>tVV\  v/ 


A CONTRIBUTION  TO  THE  CHEMISTRY  OF  THE 
TELLURATES. 


introduction. 


5 


The  first  systematic  study  of  tellurium  and  its  compounds  was 
made  by  Berzelius  in  the  early  part  of  the  last  century.  He 
showed  that  tellurium  forms  two  oxygen  acids,  H2Te03  and 
H2Te04.  Berzelius  prepared  telluric  acid  in  the  crystalline  form 
and  showed  that  it  has  the  composition  H2Te04.2H20.  He 
prepared  a large  number  of  the  salts  of  both  tellurous  and  telluric 
acids  and  found  that  nearly  all  of  the  metals  yield  insoluble  com- 
pounds with  these  acids.  Consequently  the  tellurates  were 
largely  obtained  in  the  form  of  precipitates.  Berzelius  made  quite 
a detailed  study  of  some  of  these  compounds.  In  the  case  of 
others,  however,  the  tellurate  was  prepared  and  but  little  more 
is  recorded  concerning  it.  The  work  of  Berzelius  is  of  great 
importance  in  that  it  has  not  only  furnished  us  with  a large  num- 
ber of  facts  concerning  these  compounds  but  also  paved  the  way 
for  a fuller  investigation  of  their  properties  by  later  chemists. 

Oppenheim,  Becker,  Staudenmaier,  Retgers,  Mylius,  and  Gut- 
bier  have  each  made  important  contributions  to  our  knowledge 
"of  telluric  acid  and  its  salts. 

All  of  the  alakli  metals,  sodium,  potassium,  rubidium,  and 
caesium  yield  soluble  crystalline  tellurates  and  these  have  re- 
ceived the  attention  of  investigators  to  a greater  extent  than  have 
the  tellurates  of  the  heavy  metals.  Indeed  many  of  the  tellurates 
that  were  obtained  as  precipitates  by  Berzelius  appear  not  to 
have  been  studied  at  all  by  later  investigators.  An  examination 
of  the  literature  on  the  tellurates  shows  that  detailed  knowledge 
of  most  of  them  is  wanting.  Many  of  the  properties  that  have 
been  observed  appear  to  be  those  of  mixtures.  This  is  doubtless 


3 


42 


BULLETIN  OF  THE  UNIVERSITY  OF  WISCONSIN. 


due  to  the  fact  that  telluric  acid  possesses  weak  acid  properties 
and  has  a great  tendency  to  form  basic  salts,  particularly  with 
the  heavy  metals,  and,  in  general,  to  form  complex  salts.  It  is 
frequently  found  that  the  line  of  demarcation  between  two  of 
the  tellurates  as  they  separate  from  solution  is  so  narrow  that 
one  salt  is  contaminated  by  another  and  sometimes  by^a  third  or 
a fourth.  The  tellurates  of  'the  alkali  metals  are  the  only  ones 
that  have  hitherto  been  described  as  crystalline. 

The  following  work  was  undertaken  with  a view  of  preparing 
a number  of  the  tellurates  and  studying  their  properties.  The 
work  has  been  confined  for  the  most  part  to  the  salts  of  potassi- 
um, silver,  and  mercury.  Careful  attention  has  been  devoted  to 
their  preparation  in  the  crystalline  form.  The  ease  with  which 
the  tellurates  of  silver  and  mercury  are  decomposed  by  water  has 
made  this  problem  particularly  difficult.  For  the  most  part  the 
salts  appear  at  first  as  amorphous  precipitates  of  varying  com- 
position and  quickly  alter  in  the  mother  liquor,  their  subsequent 
composition  depending  on  the  conditions  in  the  solution. 


IIUTCPIINS CHEMISTEY  OF  THE  TELLUEATES. 


4^ 


TELLURIC  ACID. 

Berzelius  1 prepared  telluric  acid  by  oxidizing  tellurium  diox- 
ide in  alkaline  solution  with  chlorine.  From  this  solution  he 
precipitated  barium  tellurate  by  means  of  barium  chloride  and 
subsequently  treated  the  barium  salt  formed,  with  sulphuric  acid. 
Sulphuric  acid  liberates  free  telluric  acid  from  barium  tellurate, 
insoluble  barium  sulphate  being  formed  at  the  same  time.  Ber- 
zelius also  oxidized  tellurium  dioxide  by  fusing  it  with  potassium 
nitrate.  From  the  potassium  tellurate  thus  formed,  he  obtained 
barium  tellurate  from  which,  in  turn,  he  liberated  the  free  acid 
by  means  of  sulphuric  acid. 

Oppenheim^  obtained  the  acid  by  fusing  tellurium  dioxide  with 
potassium  hydrate  and  potassium  chlorate.  He  used  the  method 
of  Berzelius  for  preparing  the  free  acid  from  the  alkali  tellurate. 

Becker®  dissolved  tellurium  in  nitric  acid  and  oxidized  the  solu- 
tion by  means  of  lead  peroxide.  The  lead  tellurate  obtained  was 
decomposed  by  sulphuric  acid. 

Gutbier'* *  and'Resenscheck  have  shown  that  telluric  acid  can  be 
prepared  by  oxidizing  tellurium  dioxide  by  means  of  hydrogen 
peroxide  in  a solution  which  is  strongly  alkaline  with  sodium  or 
potassium,  hydroxide.  The  alkali  tellurate  which  is  obtained  in 
this  manner  is  treated  with  nitric  acid  and  the  telluric  acid  ob- 
tained in  The  free  condition. 

The  telluric  acid  used  in  this  work  was  obtained  by  purifying 
and  oxidizing  electrolytic  tellurium  prepared  by  the  Baltimore 
Copper  Co.  This  tellurium^  contains  silver,  copper,  and  selenium 
as  its  principal  impurities.  The  acid  was  prepared  by  the  method 
described  by  Staudenmaier.®  By  this  method  free  telluric  acid  is 
obtained  by  oxidation  of  tellurous  acid  by  means  of  chromic  acid. 
In  many  respects  this  method  is  superior  to  those  in  which  sul- 
phuric acid  is  used  to  set  the  telluric  acid  free.  The  yield  is 
greater,  the  operaton  is  move  direct,  and  the  sulphuric  and  selenic 

^Pogg.  Ann.,  Vol.  32,  p.  23. 

2Jour.  f.  pr.  Chem.,  Vol.  71,  p.  266. 

3 Ann.  CEera.,  Vol.  180,  p.  256. 

*Zeit.  f.  anorg.  Chem.,  Vol.  40,  p.  260. 

’Zeit.  f.  anorg.  Chem.,  Vol.  10,  p.  189. 


44 


BULLETIN  OF  THE  UNIVERSITY  OF  WISCONSIN. 


acids  are  eliminated  in  one  of  the  first  steps  of  the  process  which 
consists  in  purifying  tellurium  dioxide.  These  acids  are  apt  to 
contaminate  telluric  acid  prepared  by  the  other  methods,  unless 
special  precautions  are  taken. 

The  method  as  carried  out  in  this  work  is  as  follows : 

Tellurium  is  dissolved  in  aqua  regia  and  the  resulting  solution 
freed  from  nitric  acid  by  evaporating  with  excess  of  hydrochloric 
acid.  After  the  hydrochloric  acid  solution  has  been  diluted  and 
filtered  the  tellurium  is  precipitated  by  means  of  sodium  acid  sul- 
phite and  repeatedly  washed  with  hot  water.  The  moist  tellurium 
is  oxidized  with  nitric  acid  and  the  resulting  basic  nitrate  purified 
by  recrystallization. 

The  purified  basic  nitrate  is  covered  with  several  times  its  bulk 
of  dilute  nitric  acid.  A little  more  chromic  acid  is  added  to  the 
solution  than  is  required  by  the  equation  3Te02+2Cr03-|-3H20 
=3HoTe04ACr203.  When  the  mixture  is  boiled  for  some 
time  the  tellurium  dioxide  is  completely  oxidized  to  telluric 
acid  according  to  the  above  equation.  The  solution  is  then  con- 
centrated over  the  water  bath  until  a crystalline  crust  begins  to 
form  and  allowed  to  cool  slowly.  The  telluric  acid  separates  as  a 
compact  crystalline  layer  which  is  easily  removed  from  the  solu- 
tion and  rinsed  to  free  it  from  the  mother  liquor.  If  the  nitric 
acid  solution  to  which  the  chromic  acid  has  been  added  is  quite 
concentrated,  telluric  acid  will  separate  from  the  hot  solution  in 
the  form  of  fine  crystals.  The  solution  is  cooled,  decanted,  and 
again  concentrated  over  the  water  bath  until  crystals  form.  This 
operation  is  repeated  as  long  as  telluric  acid  separates  from  the 
mother  liquor. 

The  telluric  acid,  which  is  green  from  the  presence  of  chromium 
nitrate,  is  dissolved  in  hot  water  and  recrystallized.  If  a white 
residue  remains  when  the  acid  is  dissolved,  it  is  an  indication  that 
the  amount  of  chromic  acid  was  insufficient  to  oxidize  all  of  the 
tellurium  dioxide.  A residue  may  remain  even  when  the  hot 
nitric  acid  solution  is  clear.  This  is  due  to  the  fact  that  tellurium 
dioxide  is  much  more  soluble  in  hot  nitric  acid  than  in  cold,  and 
so  may  separate  with  the  telluric  acid  when  the  solution  is  cooled. 
In  order  to  remove  the  chromium  nitrate  from  the  telluric  acid 
and  obtain  a pure  white  product  the  acid  must  be  recrystallized 


6 


HUTCHINS CHEMISTUY  OF  THE  TELLURATES. 


45 


from  ten  to  twenty-five  times,  depending  upon  the  amount  of  the 
chromium  salt  present. 

It  has  been  found  possible  to  prepare  fifteen  hundred  grams  of 
the  recrystallized  acid  in  six  weeks.  The  yield  is  about  So%. 
Telluric  acid  remains  in  the  mother  liquor  along  with  the  chro- 
mium nitrate,  from  which  it  cannot  be  readily  separated  as  the 
solution  becomes  syrupy  upon  evaporation. 

Telluric  acid  is  readily  soluble  in  either  hot  or  cold  water.  It 
is  much  more  soluble  in  hot  water  than  in  cold.  Its  solubility  is 
decreased  by  the  addition  of  nitric  acid.  This  fact  may  be  made 
use  of  in  the  separation  of  telluric  acid  from  chromium  nitrate. 
When  concentrated  nitric  acid  is  added  to  a concentrated  solution 
of  these  compounds  telluric  acid  is  precipitated;  the  chromium 
nitrate  being  soluble  in  nitric  acid,  remains  in  solution.  If  a solu- 
tion of  telluric  acid  is  concentrated  rapidly  it  does  not  crystallize 
but  appears  as  a syrupy  or  gummy  mass. 

Telluric  acid  is  dimorphous®  crystallizing  in  the  hexagonal- 
rhombohedral  and  isometric  systems.  It  usually  crystallizes  from 
its  solutions  in  the  rhombohedral  system  but  regular  octahedra  are 
sometimes  formed.  The  crystals  are  colorless  and  transparent. 
They  have  the  composition  represented  by  the  formula  H2Te04. 
2U^O. 

Staudemaier^  describes  a second  hydrate  of  telluric  acid  having 
the  composition  H2Te04.6H20  which  he  obtained  by  cooling 
an  aqueous  solution  of  telluric  acid  to  zero. 

Telluric  acid  is  a weak  acid  showing  but  a feeble  reaction  to 
litmus.  Gutbier®  found  that  telluric  acid  has  about  the  same 
molecular  conductivity  as  hydrocyanic  acid  or  hydrogen  sulphide. 
He  determined  the  molecular  weight  of  telluric  acid  by  the  freez- 
ing point  method.  From  the  results  which  he  obtained  he  con- 
cludes that  telluric  acid  should  be  represented  by  the  formula 
Te(OH)6  and  not  H2Te04-|-2H20. 

When  heated  above  iio°  telluric  acid  loses  water.  According 
to  Gutbier®  constant  weight  is  obtained  at  145°.  He  shows  that 
the  loss  in  weight  at  that  temperature  is  not  as  great  as  that  re- 

®Gutbier — Studien  iiber  das  Tellur,  p.  18. 

H.  c. 

81.  c.  / 

® Studien  iiber  das  Tellur.,  p.  16. 

7 


i 


46 


BULLETIN  OF  THE  UNIVEESITY  OF  WISCONSIN. 


quired  by  the  change  of  H2Te04.2H20  to  H2Te04.  Experiments 
carried  out  during  the  progress  of  this  work  have  given  similar 
results  to  those  obtained  by  Gutbier. 

Mylius^®  states  that  H2Te04.2H20  is  changed  to  (HgTeO^)  ^ 
at  about  140°.  He  calls  the  polymerized  acid  allotelluric  acid  and 
shows  that  its  solutions  have  a much  greater  conductivity  than 
the  acid  that  has  not  been  dehydrated.  The  conductivity  of  solu- 
tions of  allotelluric  acid,  according  to  Mylius,  diminishes  upon 
standing.  This  may  indicate  that  the  acid  gradually  assumes 
the  composition  HgTeOg  when  in  solution. 

While  telluric  acid  may  be  considered  a weak  acid,  a hot  aque- 
ous solution  of  it  will  attack  many  metals  such  as  silver,  mercury, 
lead,  bismuth,  copper,  zinc,  arsenic,  antimony^  tin,  aluminum,  cad- 
mium, and  nickel.  A number  of  these  metals  are  attacked  by  a 
cold  solution  of  the  acid. 


SALTS  OF  TELLURIC  ACID. 

The  types  of  salts  that  telluric  acid  forms  may  be  represented 
as  follows:  (2M20.3Te03),  (M'20.2Te03),  (M'20.Te03), 

(3M'20.2Te03),  and  (3M'20'.Te03).  Crystalline  salts  of  all  of 
these  types  are  now  known.  With  the  possible  exception  of  mer- 
curic orthotellurateii  (Hg3TeOg)  no  anhydrous  crystalline  salt 
has  been  described.  It  is  claimed  by  Nandi  and  von  Lang^^  that 
K2Te04  exists  as  a crystalline  salt  in  the  anhydrous  condition  but 
its  existence  has  been  disputed by  Retgers,  Rammelsberg,  Stau-  ^ 
denmaier,  and  Gutbier. 

Berzelius  considered  telluric  acid  a dibasic  acid  and  accord- 
ingly called  salts  of  the  type  M'2Te04  normal  tellurates  and  salts 
of  the  other  types  basic  or  acid  tellurates  according  as  they  con- 
tain more  or  less  metallic  oxide  than  the  normal  salt.  This 
nomenclature  has  been  universally  adopted  in  describing  the  tel- 
lurates and  is  adhered  to  in  the  following  description.  However, 


lOBer.,  Vol.  34,  II,  p.  2208. 

“New  salt. 

“Wiener  Akad.;  Ber.,  Vol.  43,  p.  117. 
“Gutbier,  Studien  iiber  das  Tellur.,  p.  36. 

8 


9 


HUTCHINS CHEMISTRY  OF  THE  TELLUEATES.  ' 47 

salts  of  the  type  M'gTeOe  are  termed  ortho-tellurates  from  their 
evident  relation  to  orthotelluric  acid  HgTeOg. 

We  have  both  amorphous  and  crystalline  salts  of  the  type 
M'gTeOg  such  as  lAg^TeOgd^  Hg^TeOgd^  CugTeOgd®  and  Zn^ 
TeOgd®  These  salts  appear  to  be  derived  from  the  hexabasic 
acid  HgTeOg  which  is  identical  in  composition  with  ordinary  tel 
luric  acid. 

The  crystalline  salts  of  the  type  MsO.TeOg  contain  two  or  more 
molecules  of  water^  for  example,  (Na20.Te03.2H20),^'^  (^2^- 
Te0,.2U,0')  ( Cs2O.TeO3.3H2O ) ( Rb2O.TeO3.3H2O ) 

(Ag2O.TeO3.2H2O) (Hg0.Te03.2H20).^®  These  compounds 
may  be  considered  acid  salts  of  orthotelluric  acid  of  the  types — 
(Na2H,TeOg),  (K.H.TeOg),  (Cs2H,Te06.H20'),  (Ag2H,Te 
OJ,  (HgH.TeOJ. 

Likewise  the  silver  salt  (3Ag2O.2TeO3.3H2O')  which  accord- 
ing to  the  nomenclature  in  vogue  is  a basic  salt,  may  be  consid- 
ered an  acid  salt  of  the  type  Ag3H3Te06. 

This  view  of  the  tellurates  finds  some  confirmation  in  the  ex- 
periments of  Gutbier  and  Mylius  on  the  molecular  weight  and 
electrical  conductivity  of  telluric  acid.  It  is  also  supported  by 
the  behavior  of  the  hydrous  tellurates  when  heated.  Gutbier-^ 
has  shown  that  the  alkali  tellurates  decompose  with  the  libera- 
tion of  oxygen  before  the  water  which  they  contain  is  entirely 
driven  off.  The  silver  salts  mentioned  do  not  give  up  all  of  their 
water  until  a temperature  of  200°  is  reached.  Oxygen  is  evolved 
from  these  salts  at  a temperature  only  a few  degrees  above  this 
point. 

Some  of  the  crystalline  tellurates  contain  more  water  than  is 
required  to  make  them  derivatives  of  orthotelluric  acid ; such 
salts  are  K2Te04.5H20^^  and  HHgTe04.3H20.^^  These  com- 


Amorphous,  Berzelius.  , 

‘^Crystalline,  new  salt. 

Amorphous,  new  salts. 

‘^Berzelius. 

‘SRetgers,  Zeit.  f.  phys.  Chem.,  Vol.  10,  p.  .536. 

‘9 Norris  and  Kingman,  Am.  Chem.  Jr.,  Vol.  26,  p.  320. 
29  New  salts. 

2iStudien  iiber  das  Tellur.,  p.  34. 

22  Berzelius. 

22  New  sait.  ; 


i 


9 


48 


BULLETIN  OF  THE  UNIVEKSITY  OF  WISCONSIN. 


pounds  may  be  considered  acid  salts  containing  water  of  crystal- 
lization, thus, — K2H4Te06.3H20  and  HgH5Te06.H20. 

Such  crystalline  salts  as  CsHTe04.^H20  and  RbHTeO^.^ 
H20“^  would  have  to  be  considered  salts  of  a higher  acid,  as 
H6Te209. 

Corresponding  to  the  type  M'2Te04  a large  number  of  anhy- 
drous tellurates  have  been  prepared  but  they  are  all  amorphous. 
These  salts  may  also  be  considered  metatellurates.  Their  rela- 
tion to  orthotelluric  acid  is  similar  to  that  which  the  metaborates 
bear  to  orthoboric  acid.  It  is  problematical  how  much  water 
these  salts  would  contain  were  it  possible  to  prepare  them  in  the 
crystalline  condition. 

In  addition  to  the  types  of  tellurates  enumerated  above  a num- 
ber of  tellurates  corresponding  to  M2Te207  have  been  prepared. 
They  have  been  called  the  pyrotellurates.  Berzelius  also  de- 
scribed the  salts  Na2O.4TeO3.5H2O  and  K2O.4TeO3.4H2O. 


METHODS  OF  ANALYSIS. 

Many  of  the  metals  may  be  precipitated  from  solutions  of  the 
tellurate  under  conditions  that  will  allow  all  of  the  tellurium  to 
remain  in  the  filtrate.  Thus  silver  was  determined  in  some  cases 
by  precipitation  as  chloride  from  a solution  of  the  tellurate  in 
dilute  nitric  acid. 

In  the  analysis  of  the  tellurates  of  those  metals  where  chlorides 
are  not  volatile  below  a red  heat  the  tellurium  has  been  volatil- 
ized as  Te02.2HCl  and  the  non-volatile  chloride  weighed.  This 
method  of  determining  the  metal  is  preferred  to  the  method  of 
precipitation  because  it  is  less  time-consuming;  there  are  fewer 
operations  required  for  the  analysis  and  less  opportunity  for  ex- 
perimental error.  In  cases  where  it  is  applicable,  that  is,  where 
the  chloride  of  the  metal  is  not  readily  volatile,  it  gives  at  once 
a clean  separation  of  the  metal  from  telluric  acid.  Moreover  the 
tellurium  is  obtained  in  a suitable  solution  for  precipitation  with 
sulphur  dioxide. 


24  Norris  and  Kingman,  1.  c. 


10 


i 


HUTCHINS CHEMISTRY  OF  THE  TELLURATES.  49 

Professor  Lenher^^  has  shown  that  when  dry  hydrochloric  acid 
gas  is  passed  over  a tellurite  heated  to  a temperature  somewhat 
below  redness  the  tellurium  is  completely  volatilized  as  Te02. 
2HCI.  If  the  chloride  of  the  metal  is  not  volatile  it  remains  in 
the  boat.  He  has  employed  this  method  in  the  analysis  of  a 
number  of  the  tellurites.  The  method  has  been  found  to  work 
equally  well  for  the  analysis  of  salts  of  telluric  acid.  Following 
is  a description  of  the  method  as  it  has  been  carried  out. 

A sample  of  the  tellurate  tO'  be  analyzed  is  placed  in  a porce- 
lian  boat  which  has  been  weighed.  The  boat  is  loosely  covered 
with  a thin  glass  plate  and  introduced  into  a hard  glass  tube. 
Hydrochloric  acid  gas  is  generated  by  allowing  concentrated 
hydrochloric  acid  to  drop  slowly  into  a flask  containing  concen- 
trated sulphuric  acid.  In  this  manner  a steady  stream  of  the 
gas  is  provided.  The  gas  after  having  been  dried  by  sulphuric 
acid  is  passed  through  the  tube,  which  is  heated  to'  a dull  red  heat. 
The  hydrochloric  acid  decomposes  the  tellurate  with  the  forma- 
tion of  the  chloride  of  the  metal,  chlorine,  and  Te02.2HCl  accord- 
ing to  the  following  equation  : M2Te04  + 6HC1  = 2M.CI  + 2CI 
-[-  Te02.2HCl  2H2O.  The  tellurium  compound  sublimes  upon 
the  cold  part  of  the  tube  in  the  form  of  crystalline  flakes  leaving 
only  the  chloride  of  the  metal  in  the  boat.  The  boat  is  removed 
from:  the  tube,  cooled  in  a desiccator  and  weighed.  The  escaping 
hydrochloric  acid  gas  is  collected  in  a receiver  containing  cold 
water.  The  tellurium  may  be  washed  from  the  tube  into  the 
receiver  with  hydrochloric  acid  and  precipitated  with  sulphurous 
acid.  Tellurium  was  repeatedly  determined  by  this  method. 

The  method  which  Norris  and  Kingman^®  used  in  the  analysis 
of  the  tellurates  was  largely  employed  for  the  estimation  of  tel- 
lurium. This  method  depends  upon  the  fact  that  hot  aqueous 
hydrochloric  acid  completely  reduces  telluric  acid  to  tellurium 
tetrachloride  with  the  liberation  of  free  chlorine,  according  to  the 
following  equation : H2Te04  -f  6HC1  = CI2  + TeCl^  -f  4H2O. 
This  reaction  goes  on  only  slowly,  if  at  all,  in  the  cold.  The 
analysis  is  carried  out  as  follows : 

Unpublished  notes. 

c. 


11 


i 


50 


BULLETIN  OF  THE  UNIVERSITY  OF  WISCONSIN. 


The  sample  to  be  analyzed  is  placed  in  a small  round-bottomed 
flask  together  with  a small  piece  of  magnesite.  About  fifty  cubic 
centimeters  of  concentrated  hydrochloric  acid  is  added.  The 
solution  is  heated  until  about  one-third  of  it  has  distilled  over, 
the  distillate  being  collected  in  a solution  of  potassium  iodide 
kept  cool  by  a freezing  mixture.  The  liberated  iodine  is  titrated 
with  a solution  of  sodium  thiosulphate. 

To  test  the  accuracy  of  this  method  a weighed  amount  of  re- 
crystallized telluric,  acid  was  decomposed  by  hydrochloric  acid. 
The  chlorine  liberated  by  the  reaction  was  absorbed  by  a solution 
of  potassium  iodide  and  the  liberated  iodine  titrated  with  a solu- 
tion of  sodium  thiosulphate.  The  solution  of  sodium  thiosul- 
phate was  standardized  against  pure  iodine.  0.4465  grams  of 
telluric  acid  liberated  chlorine  equivalent  to  0.2482  grams  of  tel- 
lurium. H2Te04.2H20  requires  55.56%  Te;  found  55.59%.^^ 

If  it  is  desired,  the  metal  may  be  determined  in  the  solution 
after  the  tellurate  has  been  reduced  and  the  chlorine  distilled. 
This  was  done  in  some  cases  where  the  amount  of  material  at 
hand  was  small. 

Tellurium  can  be  determined  in  telluric  acid  or  its  salts  much 
more  quickly  by  this  method  than  by  any  method  requiring  pre- 
cipitation of  elementary  tellurium.  The  entire  determination 
requires  less  than  one  hour.  Moreover  the  method  has  proven 
more  reliable  as  carried  out  in  this  work  than  precipitation  of 
elementary  tellurium  by  sulphur  dioxide. 


In  separating  mercury  from  telluric  acid  use  has  been  made 
of  the  fact  that  tellurium  sulphide  is  soluble  in  a solution  of 
ammonium  sulphide.  The  tellurate  of  mercury  is  dissolved  in 
dilute  hydrochloric  acid.  Mercurous  compounds  are  oxidized  by 
chlorine  or  bromine  water.  If  the  acid  solution  is  saturated  with 
hydrogen  sulphide  in  the  cold  only  a trace  of  telluric  acid  is  re- 
duced.^® The  solution  of  telluric  acid  is  then  decanted  from  the 


2^ All  calculations  have  been  made  on  the  basis  of  Te  = 127.6. 
2®Brauner,  Jr.  Chem.  Soc.,  Vol.  67,  p.  545. 


12 


i 


HUTCHINS CHEMISTKY  OF  THE  TELEUEATES.  51 

precipitated  HgS.  The  precipitate  is  washed  by  decantation  with 
hydrogen  sulphide  water  to  free  it  from  telluric  acid,  and  then 
digested  with  ammonium  sulphide  to  remove  the  trace  of  tellu- 
rium precipitated  with  the  mercuric  sulphide. 

Instead  of  decanting  the  solution  of  telluric  acid  from  the  pre- 
cipitated mercuric  sulphide  it  may  be  made  strongly  ammoniacal 
and  saturated  with  hydrogen  sulphide.  Telluric  acid  is  reduced 
at  once  by  hydrogen  sulphide  in  alkaline  solution.  The  precipi- 
tate that  is  formed  dissolves  immediately  in  ammonium  sulphide 
forming  a yellow  solution.  All  of  the  tellurium  will  now  be 
found  in  the  solution.  The  mercuric  sulphide  is  washed  with  a 
solution  of  ammonium  sulphide  and  hydrogen  sulphide  v/ater, 
dried  at  iio°  and  weighed. 


Water  was  determined  in  most  cases  by  heating  the  tellurate 
in  a current  of  dry  air  and  collecting  the  water  in  sulphuric  acid. 


13 


52 


BULLETIN  OF  THE  UNIVERSITY  OF  WISCONSIN. 


I 


THE  TELLURATES  OF  SILVER. 

NORMAL  SILVER  TULLURATU. 

Berzelius  describes  normal  silver  tellurate  (Ag2Te04)  as  a 
dark  yellow  precipitate.  He  states  that  he  obtained  this  com- 
pound in  the  form  of  bulky  flakes  by  adding  a concentrated  solu- 
tion of  silver  nitrate  to  a solution  of  normal  potassium  tellurate. 
If  the  precipitate  is  treated  with  boiling  water  it  is  converted  into 
a brown  basic  silver  tellurate. 

Gutbier^^  states  that  it  is  hardly  possible  to  prepare  normal 
silver  tellurate  in  pure  condition  by  the  method  described  by  Ber- 
zelius. According  to  Gutbier  the  precipitate  obtained  from  con- 
centrated solutions  of  silver  nitrate  and  potassium  tellurate  is 
always  contaminated  with  ai  larger  or  smaller  amount  of  basic 
silver  tellurate. 

It  has  not  been  found  possible  to  prepare  normal  silver  tel- 
lurate (Ag2Te04)  in  pure  condition  by  precipitation  as  described 
by  Berzelius.  When  solutions  of  silver  nitrate  and  normal  potas- 
sium tellurate  are  brought  together,  the  precipitate  contains 
basic  silver  tellurate.  The  formation  of  a basic  salt  may  be 
obviated  by  using  an  acid  tellurate  of  potassium,  for  example, 
KHTe04,  place  of  the  normal  tellurate.  But  under  these  con- 
ditions the  precipitate  may  contain  an  excess  of  telluric  acid. 
Moreover  when  the  precipitate  is  washed  it  is  decomposed  form- 
ing a basic  tellurate  of  silver. 

It  has  been  found  possible  to  prepare  well  crystallized  normal 
silver  tellurate.  Crystals  of  this  salt  are  not  rapidly  attacked  by 
cold  water  and  may  be  obtained  free  from  basic  salts. 

Silver  tellurate  has  been  obtained  by  the  following  methods : 
the  action  of  telluric  acid  upon  silver  oxide;  the  double  decom- 
position of  a soluble  silver  salt  with  a soluble  tellurate  or  telluric 
acid;  the  action  of  telluric  acid  upon  metallic  silver. 


29pogg.  Ann.  d.  Phys.  u.  Chem.,  Vol.  32,  p.  577. 
30Zeit.  f.  Anorg.  Chem.,  Vol.  31,  p.  340. 


14 


i 


HUTCHINS CHEMISTRY  OF  THE  TELLURATES.  53 


Action  of  Telluric  Acid  on  Oxide  of  Silver, 

When  silver  oxide  is  treated  with  a slight  excess  of  a solution 
of  telluric  acid,  normal  silver  tellurate  is  formed  according  to  the 
equation  : Ag20  + H3Te04.2H20  = Ag2Te04.2H20  + H2O. 

The  silver  tellurate  obtained  by  this  method  has  quite  different 
physical  properties  from  the  yellow  precipitate  described  by  Ber- 
zelius and  Gutbier.  It  is  a heavy  granular  powder  nearly  white 
in  color.  It  has  the  composition  Ag2Te04.2H20.  It  is  much 
more  stable  in  the  presence  of  water  than  the  yellow  precipitate 
hitherto  described.  When  the  powder  is  heated  to  100°  it  loses 
a part  of  its  water  and  assumes  a brown  color ; but  it  does  not 
give  up  all  of  its  water  until  a much  higher  temperature  is 
reached.  It  is  soluble  in  ammonium  hydrate,  sodium  thiosul- 
phate, and  the  acids.  The  white  powder  reacts  with  silver  oxide 
suspended  in  hot  water  to  form  a brown  basic  tellurate  of  silver. 

If  a solution  of  Ag2Te04.2H20  in  acetic  or  nitric  acid  is  evap- 
orated to  dryness  in  vacuo  telluric  acid  and  silver  acetate  or 
nitrate  are  formed.  When  the  salt  is  strongly  compressed  be- 
tween steel  rolls  it  detonates  and  assumes  a brown  color.  The 
same  property  is  possessed  by  the  tellurates  of  mercury.  This 
work  will  be  continued  later. 

For  analysis  the  salt  was  dried  over  sulphuric  acid  and  dis- 
solved in  dilute  nitric  acid.  The  silver  was  precipitated  from 
the  solution  by  dilute  hydrochloric  acid.  The  filtrate  was  evap- 
orated with  an  excess  of  hydrochloric  acid  to  free  it  from  nitric 
acid.  The  tellurium  was  precipitated  in  the  hydrochloric  acid 
solution  by  sulphurous  acid. 

Analysis  gave  the  following  results : 

0.9597  grams  gave  0.6172  grams  AgCl  and  0.2755  grams  Te. 

0.9051  grams  gave  0.0724  grams  H2O. 

Ag  Te  H2O 

Calculated  for  Ag2Te04.2H20  — 48.68%  28.77%  8.11% 

Found  48.40%  28.06%  7-99% 


15 


i 


54 


BULLETIN  OF  THE  UNIVERSITY  OF  WISCONSIN. 


Crystalline  AgoTeO ^.2H^0. 

It  has  been  found  possible  to  obtain  crystals  of  normal  silver 
tellurate  by  slowly  evaporating  a solution  made  by  bringing  to- 
gether dilute  solutions  of  telluric  acid  and  silver  acetate  contain- 
ing free  acetic  acid.  The  dilute  acetic  acid  holds  the  tellurate  of 
silver  in  solution  and  allows  it  to  crystallize  as  the  solution  is 
evaporated.  The  acetic  acid  solution  must  be  quite  dilute,  as  a 
strong  solution  of  the  acid  prevents  the  formation  of  silver  tellu- 
rate. 

When  solutions  of  silver  acetate  and  telluric  acid  of  moderate 
concentration  are  brought  together  a lemon  yellow  precipitate  is 
formed.  The  precipitate  closely  approximates  Ag2Te04  in  com- 
position. When  a solution  of  silver  acetate  containing  2.0  grams 
of  silver  acetate  and  a few  drops  of  free  acetic  acid  per  liter,  is 
mixed  with  an  equal  volume  of  a solution  of  telluric  acid  con- 
taining 1.4  grams  of  the  acid  per  liter,  nO'  precipitate  is  formed. 
When  such  a solution  is  allowed  to  evaporate  in  the  dark  at  room 
temperature  yellow  crystals  of  Ag2Te04.2H20  are  formed.  The 
free  telluric  acid  in  the  solution  prevents  the  decomposition  of  the 
normal  salt  by  the  water.  Only  small  amounts  of  the  salt  have 
been  prepared  in  this  manner.  Larger  quantities  of  the  salt  are 
more  conveniently  prepared  in  the  following  manner : 

upon  adding  an  excess  of  a solution  of  silver  nitrate  to  a con- 
centrated solution  of  potassium  tellurate  containing  a little  free 
acetic  acid  a,  yellow  or  brown  precipitate  is  formed.  Analysis 
showed  that  this  precipitate  approximates  normal  silver  tellurate 
in  composition.  Its  exact  composition  depends  largely  on  the 
concentration  of  the  solution.  If  was  observed  that  when  such 
a solution  is  allowed  to  remain  in  contact  with  the  precipitate  for 
a few  hours  a heavy  yellow  salt  separates  at  the  bottom  of  the 
beaker.  The  salt  is  composed  of  distinct  crystals  of  a straw  yel- 
low color.  As  the  crystals  have  a much  higher  specific  gravity 
than  the  amorphous  precipitate,  they  can  easily  be  separated  from 
it  by  elutriation.  If  the  precipitate  is  allowed  to  remain  in  the 
mother  liquor  for  several  days,  red  crystals  of  a basic  silver  tel- 

16 


( 


HUTCHINS— CHEMISTKY  OF  THE  TEELURATES. 


55 


9 


lurate  will  usually  be  found  among  the  yellow  crystals  of  the 
normal  salt. 

Analysis  of  the  yellow  crystals  gave  the  following  results : 

0.7638  grams  gave  0.0605  grams  H2O. 

0.4847  grams  gave  0.31 17  grams  AgCl. 

0.4980  grams  when  heated  with  hydrochloric  acid  gave  suf- 
ficient chlorine  gas  to  liberate  0.2828  grams  of  iodine  from  potas- 
sium iodide. 

H2O  Ag  Te 

Calculated  for>g2Te04.2H20  8.11%  48.68%  28.77% 

Found  7.92%  48.41%  28.57% 

In  this  analysis  the  silver  was  precipitated  with  hydrochloric 
acid  from  a dilute  nitric  acid  solution  of  the  salt. 

Silver  acetate  or  sulphate  may  be  used  instead  of  silver  nitrate 
for  the  preparation  of  the  salt  by  double  decomposition  with 
potassium  tellurate  and  subsequent  change  of  the  amorphous  pre- 
cipitate to  the  crystalline  salt.  Likewise  the  free  acetic  acid  may 
be  replaced  by  nitric  acid,  or  the  acid  may  be  omitted  altogether. 
The  salt  crystallizes  much  better,  however,  when  the  solution 
contains  free  acid. 

Ag2Te04.2H20  crystallizes  in  the  orthorhombic  system.  The 
crystals  resemble  those  of  sulphur  both  in  the  forms  that  occur 
and  in  the  axial  ratios.  They  are  insoluble  in  hot  or  cold  water 
but  soluble  in  ammonia,  potassium  cyanide,  sodium  thiosulphate, 
and  nitric,  acetic  and  sulphuric  acids.  Concentrated  nitric,  acetic, 
or  sulphuric  acids  completely  decompose  silver  tellurate,  giving 
free  telluric  acid  and  the  corresponding  silver  salt. 

When  crystals  of  the  normal  salt  are  allowed  to  remain  for 
some  time  in  contact  with  a cold  solution  of  a silver  salt,  red'  crys- 
tals of  the  basic  tellurate  3Ag2O.2TeO3.3H2O  are  formed.  The 
change  is  much  more  rapid  in  a ten  per  cent  solution  of  silver 
nitrate  at  a temperature  of  50°  than  in  the  cold.  It  takes  place 
very  slowly  even  in  cold  water. 

Crystals  of  Ag2Te04.2H20  darken  slowly  when  exposed  to 
the  light.  When  the  powdered  crystals  are  compressed  between 
steel  rolls  they  detonate.  When  heated  above  100°  the  crystals 
lose  water  and  take  on  a brilliant  purplish  black  lustre.  They 
maintain  this  lustre  until  a temperature  of  about  200°  is  reached. 

IT 


4 


56 


BULLETIN  OF  THE  UNIVERSITY  OF  WISCONSIN. 


As  the  temperature  is  raised  still  higher  oxygen  is  evolved,  the 
mass  assumes  a gray  brown  color  and  fuses.  If  the  mass  is  al- 
lowed to  cool  as  soon  as  it  fuses,  it  becomes  black  and  crystalline, 
but  if  it  is  heated  to  bright  redness  it  is  completely  changed  to 
silver  tellurite,  which  is  white  when  cold. 

The  crystalline  normal  salt  may  be  prepared  entirely  free  from 
basic  silver  tellurate  by  allowing  the  precipitate  obtained  by  mix- 
ing solutions  of  silver  nitrate  and  acid  potassium  tellurate  to  re- 
main in  the  mother  liquor  until  it  assumes  the  crystalline  form. 

Ag2Te04.2H20 

Orthorhombic.  Axes — a :b  :c  = 0.722  :i  12.107.^^ 
pp'".  III  : iTi  = iii°  26'. 
pp'.  III  :Tii—  77°  25'. 
cp,  001  : III  = 105°  40'. 
cn,  001  :oii  = 115°  29'. 
cs,  001  : 113  = 103°  02'. 


3^Tlie  author  is  indebted  to  Professor  Hobbs  for  his  kindly  advice  in  that 
part  of  this  work  which  deals  with  the  crystallography  of  the  tellurates. 


18 


IIUTCHINS CI-IEMISTRY  OF  THE  TELEURATES. 


57 


The  crystals  that  were  used  for  measurement  were  obtained  by 
double  decomposition  of  solutions  of  silver  nitrate  and  potas- 
sium tellurate  containing  a little  free  acetic  or  nitric  acid.  The 
amorphous  precipitate  that  formed  on  mixing  the  solutions 
slowly  dissolved  in  the  free  acid,  while  at  the  same  time  crystals 
of  Ag2Te04.2H20  were  formed  upon  the  walls  of  the  containing 
vessel. 

In  order  to  determine  whether  silver  sulphate  and  silver  tel- 
lurate are  isomorphous,  crystals  of  AgoTe04.2H20  were  intro- 
duced into  a saturated  solution  of  silver  sulphate  and  the  solution 
allowed  to  evaporate.  Silver  sulphate  crystallized  out  of  the 
solution  entirely  independent  of  the  silver  tellurate.  Silver  sul- 
phate (Ag2S04)  and  silver  tellurate  ( Ag2Te04.2H20)  are  there- 
fore not  isomorphous.  It  may  be  stated  that  the  tellurate  of  sil- 
ver was  decomposed  by  the  solution  to  some  extent,  a few  red 
crystals  of  a basic  tellurate  being  formied. 

The  sulphate  and  selenate  of  silver  are  isomorphous,  their 
axial  ratios  being  as  follows : 

Silver  sulphate  a :b  :c  = 0.5712  :i  : 1.238 

Silver  selenate  a :b  :c  = 0.5945  : 1 : 1.256 

However,  these  salts  cannot  properly  be  compared  with  the 
tellurate  of  silver  since  they  are  anhydrous,  while  silver  tellurate 
contains  two  molecules  of  water. 


In  addition  to  the  tellurate  of  silver  described  by  Berzelius  Op- 
penheim^^^  describes  a crystalline  precipitate,  which  he  obtained 
in  small  quantity  by  adding  a saturated  solution  of  telluric  acid 
to  a concentrated  solution  of  silver  nitrate.  He  states  that  it  dif- 
fers from  any  tellurate  of  silver  obained  by  Berzelius  in  that  it  is 
crystalline  and  colorless.  When  exposed  to  the  air  it  assumed 
a yellow  color,  according  to  Oppenheim.  He  states  that  he  did 
not  make  a quantitative  analysis  of  the  substance  but  concluded 
that  it  was  a double  compound  of  silver  tellurate  and  nitrate, 
since  he  found  that  it  contained  nitric  and  telluric  acids  together 
with  silver. 


32Jr.  pr.  Chem.,  Vol.  71,  p.  266. 


19 


58 


BULLETIN  OF  THE  UNIVERSITY  OF  WISCONSIN. 


Gutbier^^  attempted  to  prepare  the  crystalline  compound  de- 
scribed by  Oppenheim.  When  a concentrated  solution  of  telluric 
acid  is  added  to  a concentrated  solution  of  silver  nitrate  a red 
brown  or  yellow  precipitate  is  formed.  This  precipitate  is  the 
only  product  that  Gutbier  obtained  from  such  solutions. 

When  a solution  of  silver  nitrate  is  added  to  a dilute  solution 
of  telluric  acid  no  precipitate  is  formed.  But  if  the  solutions 
are  sufficiently  concentrated  and  an  excess  of  telluric  acid  added, 
a yellow  flocculent  precipitate  is  formed  in  small  quantity.  If 
the  solution  and  precipitate  are  allowed  to  remain  together  in  a 
dark  place  at  room  temperature  for  a few  hours  the  amorphous 
precipitate  changesi  to  straw  yellow  crystals  of  Ag2Te04.2H20. 
This  is  the  crystalline  compound  that  Oppenheim  obtained.  Un- 
der the  conditions  of  the  experiment  the  crystals  might  easily  be 
contaminated  by  silver  nitrate. 

ACID  TELLURATKS  OF  SILVER. 

Berzelius  states  that  concentrated  solutions  of  KHTe04  and 
K2'I'e4043  react  with  concentrated  solutions  of  silver  nitrate  to 
form  the  corresponding  tellurate  of  silver.  If  the  solution  con- 
taining the  precipitate  is  evaporated  to^  dryness,  a white  earthy 
residue  remains.  Berzelius  states  that  the  acid  tellurates  are 
transformed  to  a brown  basic  tellurate  of  silver  in  dilute  solutions. 

Attempts  were  made  to  prepare  the  acid  tellurate  of  silver  de- 
scribed by  Berzelius.  It  was  not  found  possible  to  obtain  pre- 
cipitates having  either  the  composition  AgHTe04  or  Ag2Te40j3. 
The  precipitates  obtained  in  the  manner  described  by  Berzelius 
contain  more  telluric  acid  than  is  required  by  normal  silver  tel- 
lurate (Ag2Te04),  but  upon  remaining  in  the  mother  liquor  for 
some  hours  they  are  largely  transformed  to  crystalline  Ag2Te04. 
2H2O.  It  would  hardly  be  expected  that  a solution  of  KHTe04 
would  yield  AgHTe04  with  silver  nitrate,  when  it  is  known  that 
telluric  acid  and  silver  nitrate  give  the  crystalline  normal  tellurate. 

When  concentrated  solutions  of  KHTe04  and  silver  nitrate  are 
brought  together  a bright  yellow  precipitate  is  formed.  The 


331.  C. 


20 


I 

HUTCHINS CHEMISTRY  OF  THE  TELLURATES.  5'J 

lemon  yellow  color  of  the  amorphous  precipitate  is  permanent, 
there  being  no  tendency  to  form  a brown  basic  tellurate  in  the 
medium  present.  Analysis  of  the  precipitate  obtained  in  this 
manner  showed  that  it  contained  46.6%  silver  and  32.9%  tel- 
lurium. The  ratio  between  the  silver  and  telluric  acid  in  the  pre- 
cipitate lies  between  those  required  by  the  formulae  Ag2Te04  and 
AgFlTeO^.  Some  of  the  precipitate  was  allowed  to  remain  in 
the  mother  liquor  for  forty-eight  hours.  At  the  end  of  that  time 
a large  part  of  it  had  been  converted  into  crystalline  normal  sil- 
ver tellurate. 

When  a concentrated  solution  of  silver  nitrate  is  added  to  a 
solution  of  acid  potassium  tellurate  made  by  dissolving  potas- 
sium hydrate  and  telluric  acid  together  in  the  proportions  re^ 
quired  by  the  formula  K2'I'^40i3  ^ bright  yellow  precipitate  is 
formed.  When  left  in  contact  with  the  mother  liquor  for  some 
time  the  precipitate  is  largely  changed  to  crystalline  Ag2Te40. 
2H2O. 

Analysis  of  the  crystalline  salt  obtained  in  this  manner  gave 
the  following  results : 

0.4006  grams  gave  0.0329  grams  H2O  and  0.2571  grams  AgCl. 

0.5384  grams  with  HCl  liberated  chlorine  equivalent  to  0.1555 
grams  Te. 

Found  H2O,  8.21%;  Te,  28.88%;  Ag,  48.31%. 

The  silver  tellurate  prepared  in  this  m.anner  is  not  well  crys- 
tallized. This  is  due  to  the  fact  that  the  precipitate  is  only 
slightly  soluble  in  the  mother  liquor.  The  crystals  darken  only 
very  slowly  in  direct  sunlight. 

When  solutions  of  HKTe04  and  silver  nitrate  are  brought 
together  in  equimolecular  proportions  and  the  whole  evaporated 
to  dryness  a white  residue  rem.ains.  Analysis  of  the  residue  ob- 
tained in  this  manner  and  washed  with  a little  hot  water  shows 
that  it  contains  telluric  acid  in  excess  of  that  required  by  normal 
silver  tellurate.  The  residue  also  contains  potassium.  It  is  not 
possible  to  determine  how  much  of  the  telluric  acid  is  combined 
with  the  potassium,  or  how  much  of  it  is  in  the  form  of  the  free 
acid.  Consequently  no  conclusion  can  be  reached  as  to  the  exact 
composition  of  the  silver  tellurate  in  the  residue.  It  is  not  feas- 


21 


60 


BULLETIN  OF  THE  UNIVEESITY  OF  WISCONSIN. 


ible  to  wash  it  thoroughly  because  of  the  decomposition  that  sil- 
ver teilurate  undergoes  in  the  presence  of  water. 

When  crystals  of  normal  or  basic  silver  teilurate  are  boiled  with 
a concentrated  solution  of  telluric  acid,  a bright  yellow  amorphous 
precipitate  is  formed  which  is  similar  to  the  one  obtained  from 
solutions  of  acid  potassium  teilurate  and  silver  nitrate. 


BASIC  SILVUR  TULLURATES. 

Berzelius  describes  two  basic  tellurates  of  silver,  3Ag20. 
2Te03  both  of  which  were  obtained  as  brown 

precipitates.  He  states  that  boiling  water  decomposes  the  nor- 
mal teilurate  of  silver  into  3Ag20.2Te03  and  telluric  acid. 

When  a dilute  solution  of  silver  nitrate  is  added  to  a dilute 
solution  of  potassium  teilurate,  the  dark  yellow  precipitate  which 
is  first  formed  quickly  assumes  a dark  browir  color.  Berzelius 
found  this  dark  brown  precipitate  to  be  3Ag20.Te03.  He  also 
prepared  this  compound  by  adding  an  ammonical  solution  of  sil- 
ver nitrate  to  a solution  of  normal  silver  teilurate  in  ammonia 
and  evaporating  the  solution.  The  basic  teilurate  separates  from 
the  solutions  as  a brown  black  precipitate. 

Gutbier®®  states  that  the  precipitate  formed  by  the  action  of 
silver  nitrate  on  potassium  teilurate  may  be  completely  changed 
to  3Ag20.Te03  by  long  continued  washing  with  cold  water.  He 
states  that  when  the  precipitate  is  washed  with  hot  water  the  final 
product  is  3Ag20.2Te03.  According  to  Gutbier  this  salt  may 
be  prepared  by  bringing  together  hot  dilute  solutions  of  silver 
nitrate  and  potassium  teilurate. 


3Ag.0.2Tc0^,3H^0. 

In  addition  to  the  brown  precipitates  of  3Ag20.2Te03  and 
3Ag20.Te03  already  described,  a crystalline  teilurate  of  silver 
having  the  composition  3Ag2O.2TeO3.3H2O  was  prepared  dur- 
ing the  progress  of  this  work.  When  the  precipitate  obtained  by 


( 


c. 

351.  C. 


22 


HUTCHINS — CHEMISTUY  OF  THE  TELLUEATES. 


61 


the  double  deconiiposition  of  moderately  dilute  solutions  of  sil- 
ver nitrate  and  potassium  tellurate  assumes  the  crystalline  con- 
dition, both  yellow  crystals  of  Ag2Te04.2H20  and  red  crystals 
of  3Ag2O.2TeO3.3H2O  are  formed.  The  relative  amounts  of 
these  salts  obtained  depends  largely  upon  the  temperature  of  the 
solutions  and  their  concentration.  When  the  solutions  are  cold 
and  concentrated  but  little  of  the  basic  salt  is  formed.  On  the 
other  hand,  if  the  solutions  are  very  dilute  and  warm,  it  is  pos- 
sible to  obtain  the  crystalline  basic  tellurate  free  from  the  nor- 
mal salt. 

Analysis  of  the  red  crystals  gave  the  following  results : 

0.2812  grams  gave  0.0653  grams  Te 
0.3763  grams  gave  0.2917  grams  AgCl 
1.6094  grams  gave  0.0797  grams  H2O 

Te  Ag  H2O 
Calculated  for  3Ag2O.2TeO3.3H2O  23.18%  58.83%  4.90% 
Found  23.22%  58.35%  4.95% 

3Ag2O.2TeO3.3H2O  crystallizes  in  ruby  red,  transparent  crys- 
tals belonging  to  the  monoclinic  system.  The  crystals  remain 
unchanged  in  cold  water.  They  are  gradually  decomposed  by 
boiling  water  with  the  formation  of  a brown  residue,  probably 
d^g'^O-TeOg.  When  boiled  with  an  excess  of  a concentrated 
solution  of  telluric  acid  a bright  yellow  amorphous  precipitate  re- 
sults which  probably  contains  silver  bi-tellurate  (AgHTeO^). 
'fhe  crystals  are  soluble  in  the  same  reagents  that  dissolve  nor- 
mal silver  tellurate.  In  the  sunlight  the  crystals  become  opaque 
and  assume  a copper  color.  When  heated  above  110°  they  lose 
water  and  become  black,  but  all  of  the  water  is  not  given  up  until 
a temperature  of  about  200°  is  reached. 

Just  as  it  is  possible  to  obtain  crystals  of  hydrous  normal  sil- 
ver tellurate  by  evaporation  of  solutions  of  silver  tellurate  under 
proper  conditions,  so  it  is  possible  by  varying  the  conditions  to 
obtain  crystals  of  3Ag2O.2TeO3.3H2O  directly  from  clear  solu- 
tions. If  a dilute  solution  of  potassium  tellurate  is  added  to  a 
large  excess  of  silver  nitrate  containing  a little  free  nitric  acid, 
the  solution  filtered  and  allowed  to  evaporate  at  room  tempera- 
ture, red  crystals  of  the  basic  salt  are  formed.  The  formation  of 
crystals  when  the  solution  is  evaporated  is  made  possible  by  the 


62 


BULLETIN  OF  THE  UNIVERSITY  OF  WISCONSIN. 


fact  that  the  dilute  nitric  acid  is  saturated  with  silver  tellurate 
dissolved  from  the  amorphous  precipitate.  If  the  solution  con- 
tains much  nitric  acid  the  silver  tellurate  is  entirely  decomposed 
with  the  formation  of  silver  nitrate  and  telluric  acid  so  that  no 
crystals  of  the  tellurate  of  silver  appear  when  the  solution  is  evap- 
orated. Consequently  a large  volume  of  solution  is  necessary  in 
order  to  obtain  a small  amount  of  the  crystalline  salt.  Crystals 
of  3Ag2O.2TeO3.3H2O  have  been  repeatedly  obtained  in  this 
manner,  but  only  in  small  quantities. 

Larger  quantities  of  the  salt  have  been  prepared  by  double  de- 
composition of  solutions  of  silver  nitrate  and  potassium  tellurate 
and  subsequent  change  of  the  amorphous  precipitate  thus  obtained 
into  the  crystalline  salt.  If  the  solution  contains  no  free  acid  the 
crystals  form  only  in  contact  with  the  amorphous  precipitate,  since 
silver  tellurate  is  insoluble  in  water.  If,  however,  the  solution 
contains  a little  free  nitric  or  acetic  acid,  red  crystals  of  3Ag20. 
2Te03.3H20  frequently  form  on  the  walls  of  the  containing  ves- 
sel. By  working  with  dilute  solutions  at  temperatures  from  15° 
to  50°  and  using  a large  excess  of  silver  nitrate,  it  has  been 
found  possible  to  obtain  the  red  crystalline  salt  entirely  free  from 
yellow  crystals  of  the  normal  salt.  In  most  cases,  however,  more 
or  less  of  the  normal  salt  was  formed.  Both  of  the  salts  appear 
in  distinct  crystals.  Their  colors  are  so  different  that  it  is  easy 
to  ascertain  when  one  salt  contaminates  the  other  and  to  remove 
it  mechanically  if  it  is  present  in  small  amount. 


HUTCHIN-S CHEMISTEY  OF  THE  TELLURATES. 


63 


3Ag2O.2TeO3.3H2O. 

Monoclinic.  Axes — a ;b  :c  = o.5290  : i :o.27o6.  /3  = 77°  12'. 
mm'",  no  :iTo=I25°  24'. 

11,  III  : III  = 154°  28'. 

nn'",  210  ;2To=:  150°  00'. 

rb,  13T  :oio=i27°  43'. 

vb,  12T  : 010  =118°  00'. 

, ub,  iiT  :oio=  104°  33'. 

zz'",  230  1230=  104°  30'. 


25 


64 


BULLETIN  OF  THE  UNIVEESITY  OF  WISCONSIN. 


SAg^O.TeO^. 

Silver  orthotellurate  (3Ag20.Te03)  has  not  been  obtained  in 
the  crystalline  form.  It  is  known  only  in  the  form  of  the  brown 
amorphous  precipitate  described  by  Berzelius.  The  salt,  some- 
what contaminated  by  3Ag20.Te03,  was  obtained  by  adding  a 
hot  dilute  solution  of  potassium  tellurate  to  a solution  of  silver 
nitrate  containing  free  nitric  acid.  Analysis  of  the  precipitate, 
which  had  been  thoroughly  washed  with  hot  water  and  dried  over 
sulphuric  acid,  gave  72.9%  silver.  3Ag20.Te03  requires  74.3% 
silver. 

The  filtrate  was  allowed  to  evaporate  at  room  temperature. 
Crystals  of  3Ag2O.2TeO3.3H2O  formed  in  the  solution. 


1.  The  precipitate  obtained  by  double  decomposition  of  a silver 
salt  with  telluric  acid  or  a soluble  tellurate,  may  vary  in  compo- 
sition from  a mixture  containing  more  telluric  acid  than  Ag2Te04 
to  3Ag20.Te03,  depending  upon  the  conditions  in  the  medium'. 

2.  It  has  not  been  found  possible  to  obtain  the  acid  tellurates 
of  silver,  AgHTe04  and  Ag20.4Te03  described  by  Berzelius. 

3.  Silver  forms  two  crystalline  tellurates — Ag2Te04.2H20, 
which  appears  as  yellow  crystals  belonging  to  the  orthorhombic 
system,  and  3Ag2O.2TeO3.3H2O,  whose  crystals  are  red  and  be- 
long to  the  monoclinic  system. 

4.  The  crystalline  compound  that  Oppenheim  described  as  a 
double  salt  of  silver  tellurate  and  silver  nitrate,  is  Ag2Te04.2H20. 

5.  No  sulphates  or  selenates  of  silver  have  been  prepared  that 
are  analogous  to  the  crystalline  tellurates  of  silver. 


26 


HUTCHINS CHEMISTEY  OF  THE  TELEUEATES. 


G5 


THE  TELLURATES  OF  POTASSIUM. 

NORMAL  POTASSIUM  TULLURATE. 

Berzelius^®  states  that  normal  potassium  tellurate  can  be  pre- 
pared by  treating  potassium  carbonate  with  telluric  acid.  These 
compounds  were  dissolved  together  in  equimolecular  quantities 
and  the  solution  evaporated  to  dryness.  The  residue  was  dis- 
solved in  water  and  the  solution  evaporated  in  vacuo  over  sul- 
phuric acid  until  a crystalline  crust  formed.  According  to 
Berzelius  this  crust  consisted  of  normal  potassium  tellurate.  If 
the  solution  is  evaporated  at  a moderate  temperature  a gummy 
mass  is  deposited. 

Gutbier^’^  states  that  he  was  not  able  to  obtain  normal  potas- 
sium tellurate  by  this  method. 

Experiments  were  undertaken  to  ascertain  whether  normal 
potassium  tellurate  could  be  obtained  by  this  method.  It  was 
found  that  solutions  made  by  dissolving  equivalent  quantities  of 
potassium  carbonate  and  telluric  acid  in  water  always  yield  a 
gummy  mass  when  they  are  evaporated  in  vacuo. 

Potassium  tellurate  crystallizes  from  its  solutions  if  they  are 
allowed  to  evaporate  very  slowly.  This  separation  of  the  crystal- 
line salt  is  aided  by  the  addition  of  a few  crystals  of  the  salt. 
When  solutions,  made  according'  to  the  directions  of  Berzelius, 
are  evaporated  in  this  manner  no  crystals  are  formed.  The 
gummy  mass  even  separates  around  the  crystals  of  K2Te04.5H20, 
introduced  into  the  solution.  This  pasty  mass  evolves  carbon 
dioxide  when  treated  with  hydrochloric  acid.  The  same  result 
is  obtained  if  the  solution  is  cooled  to  zero  instead  of  evaporated. 

That  all  of  the  carbon  dioxide  appearing  in  the  gummy  mass 
had  not  been  absorbed  from  the  air  is  shown  by  the  following  ex- 
periment: 1.42  grams  of  anhydrous  potassium  carbonate  and 

-•37  grams  of  telluric  acid  were  dissolved  in  a little  water.  The 
solution  was  at  once  evaporated  to  dryness  over  a free  flame. 


scpogg,  Ann.,  Vol.  32,  p.  579. 

2^Zeit.  f.  anorg.  Chem.,  Vol.  31,  p.  340. 

27 


6G 


BULLETIN  OF  THE  UNIVERSITY  OF  WISCONSIN. 


The  residue  copiously  evolved  carbon  dioxide  when  treated  with 
hydrochloric  acid.  It  is  evident  that  telluric  acid  cannot  replace 
all  of  the  carbonic  acid  in  an  equimolecular  quantity  of  potas- 
sium carbonate.  This  fact  is  in  harmony  with  the  fact,  observed 
by  Berzelius,  that  carbon  dioxide  reacts  on  normal  potassium 
tellurate  to  form  acid  potassium  tellurate,  KHTeO^,  and  potas- 
sium carbonate. 

No  carbon  dioxide  is  evolved  when  saturated  solutions  of  acid 
potassium  tellurate  (KHTeO^)  and  potassium  carbonate  are 
brought  together  at  95°  in  the  proportion  of  two  molecules  of 
the  acid  tellurate  to  one  of  the  carbonate.  KHTeO^  is  more  sol- 
luble  in  a solution  of  potassium  carbonate  than  in  water. 


K^TcO,.5H^O. 

Berzelius^®  prepared  crystalline  K2Te04.5H20  from  concen- 
trated solutions  of  potassium  hydrate  and  telluric  acid  in  the  fol- 
lowing manner : A solution  of  potassium  hydrate  was  added  to 
a solution  of  telluric  acid  until  a permanent  precipitate  formed, 
indicating  the  presence  of  an  excess  of  the  alkali.  The  gummy 
precipitate  was  dissolved  by  warming  the  solution.  Crystals  of 
K2Te04.5H20  separated  from  the  solution  upon  cooling  it  to 
zero.  The  salt  is  decomposed  by  carbon  dioxide  with  the  for- 
mation of  KHTe04  potassium  carbonate. 

Gutbier^^  obtained  K2Te04.5H20  in  the  form  of  long  slender 
needles  by  employing  the  method  of  preparation  used  by  Berze- 
lius. Gutbier  states  that  it  is  very  difficult  to  obtain  the  salt  free 
from  adhering  alkali  since  it  cannot  be  recrystallized  without  add- 
ing free  alkali  to  the  solution  of  the  salt. 

Retgers^®  describes  a second  crystalline  tellurate  of  potassium 
(K2Te04.2H20)  which  he  prepared  by  dissolving  telluric  acid 
in  a concentrated  boiling  solution  of  potassium  hydroxide  and  al- 
lowing the  solution  to  cool  slowly.  He  states  that  crystals  of 
K2'f"e04.2H20  are  isomorphous  with  those  of  K2OSO4.2H2O. 


sn.  c. 

30  1.  C. 

^oZeit.  f.  phys.  Chem.,  Vol.  10,  p.  536. 

28 


HUTCHIIs^S CHEMISTHY  OF  THE  TEELUEATES. 


67 


Gutbier^^  also  prepared  by  dissolving  telluric 

acid  in  hot  concentrated  potassium  hydroxide  and  cooling  the  so- 
lution. He  found  the  salt  to  be  isomorphous  with  K2OSO4.2H2O. 

Nandi  and  von  Lang^^  describe  crystalline  anhydrous  potas- 
sium tellurate  (K2Te04)  which  they  found  to  be  isomorphous 
with  potassium  sulphate.  Gutbier,  Rammelsberg,  Staudenmaier, 
and  Retgers  have  been  unable  to  prepare  the  anhydrous  crystal- 
line tellurate.'^® 

K2'^'eO4.5H20  may  be  obtained  in  the  form  of  crystals  by  slow 
evaporation  of  solutions  of  the  salt  at  room,  temperatures.  When- 
ever solutions  of  potassium  tellurate  are  allowed  to  evaporate  in 
vacuo  over  sulphuric  acid  they  solidify  to  a gummy  mass  which 
does  not  become  crystalline  until  it  is  dry.  In  order  to  bring 
about  slower  evaporation  oi  the  solution  the  loosely  covered  dish 
containing  the  solution  may  be  placed  in  a desiccator  over  sul- 
phuric acid.  The  desiccator  should  be  filled  with  air  freed  from 
carbon  dioxide  to  prevent  the  formation  of  potassium  carbonate 
and  acid  potassium  tellurate.  Potassium  tellurate  has  been  re- 
peatedly crystallized  from  its  solutions  in  this  manner.  The  solu- 
tion may  be  concentrated  rapidly  until  its  specific  gravity  is 
about  1.2  and  then  crystallized  by  slow  evaporation.  The  fact 
that  potassium  tellurate  is  very  soluble  renders  it  somewhat  dif- 
ficult to  obtain  a nicely  crystallized  product.  Frequently  the 
crystals  form  radiating  clusters  but  in  other  cases  the  crystals  are 
distinct.  Solutions  of  potassium  tellurate  that  have  become 
supersaturated  can  frequently  be  made  to  crystallize  by  adding  a 
few  crystals  of  1x2^004. 5H2O  and  preventing  further  evaporation 
by  closely  covering  the  solution.  Even  under  these  conditions 
crystallization  proceeds  slowly. 

Crystals  of  K2Te04.5H20  may  be  obtained  by  slow  evapora- 
tion of  the  solution  resulting  from  the  double  decomposition  of 
equivalent  quantities  of  normal  silver  tellurate  and  potassium 
bromide  solution.  This  may  be  readily  accomplished  in  the  fol- 
lowing manner.  The  potassium  bromide  and  silver  tellurate  are 


^^Wiener  Akad.  Ber.,  Vol.  43,  p.  117. 
*®Gutbier,  Studien  uber  das  Tellur.,  p.  36. 


29 


68 


BULLETIN  OF  THE  UNIVEESITY  OF  WISCONSIN. 


thoroughly  ground  together,  treated  with  a little  warm  water  and 
again  triturated.  After  standing  for  some  time  in  an  atmosphere 
free  from  carbon  dioxide  the  solution  is  filtered  and  crystallized 
in  the  manner  described  above. 

Analysis  of  crystals  obtained  in  this  manner  gave  the  follow- 
ing results : 

0.9742  grams  gave  0.2420  grams  H2O. 

0.9742  grams  gave  0.4010  grams  KCl. 

0.2839  grams  with  hydrochloric  acid  liberated  chlorine  equiv- 
alent to  o.iooi  grams  Te. 


Calculated  for  K2Te04.5H20 

Found 

H20 

25.01 

24.84 

K 

21.76 

21.60 

Te 

3545 

35-26 

Crystals  of  K2Te04.5H20  are  soft  and  crumble  under  a slight 
pressure.  Solutions  of  the  salt  are  alkaline  to  litmus  and  attack 
glass. 

Solutions  of  potassium  tellurate  containing  two  molecules  of 
free  potassium  hydroxide  to  one  of  the  tellurate  yield  crystals  of 
K2"re04.5H20  when  evaporated  slowly.  The  yield  is  small  on 
account  of  the  free  alkali  in  the  solution.  Solutions  of  this  kind 
were  evaporated  in  order  toi  ascertain  whether  potassium  tellurate 
would  crystallize  with  two  or  five  molecules  of  water  under  these 
conditions. 

Crystals  that  had  been  washed  with  water  and  dried  with  filter 
paper  gave  the  following  results  upon  analysis : 

1.1936  grams  gave  0.3000  grams  H2O. 

1.1936  grams  gave  0.4916  grams  KCl. 

0.6443  grams  with  hydrochloric  acid  liberated  chlorine  equiv- 
alent to  0.2306  grams  Te. 


Calculated  for  K2Te04.5H20 

Found 

H2O 

25.01 

25-15 

K 

21.75 

21.63 

Te 

3545 

35-79 

30 


HUTCHINS CHEMISTHY  OF  THE  TELLURATES. 


69 


K,Te0,.2H^0. 

This  salt  was  obtained  in  the  form  of  distinct  crystals  by  dis- 
solving telluric  acid  in  a hot  concentrated  solution  of  potassium 
hydrate  and  allowing  the  solution  to  cool.  The  yield  was  very 
small. 

Analysis  of  the  salt  after  having  been  washed  with  water  at 
zero  and  dried  for  a day  over  calcium  chloride  gave  the  follow- 
ing result: 

1.4806  grams  gave  0.1767  grams  of  water. 

Calculated  for  K2Te04.2H20  ii47 

Found  1 1 .93 

When  the  solution  of  potassium  hydrate,  from  which  the  crys- 
tals of  K2Te04.2H20^  had  been  removed,  was  diluted  a copious 
amount  of  K2Te04.5H20  separated  as  small  crystals.  This  fact 
indicates  that  the  formation  of  the  dihydrate  in  the  concentrated 
solution  is  due  to  the  dehydrating  effect  of  the  strong  alkali.  It 
is  also  evident  that  K2Te04.5H20  is  less  soluble  in  a dilute  solu- 
tion of  potassium  hydrate  than  K2Te04.2H20  is  in  a concentrated 
solution. 


ACID  Te:uUURATES  OF  POTASSIUM. 

Berzelius  states  that  potassium  bitellurate  (KHTe04)  may  be 
obtained  from  equivalent  quantities  of  telluric  acid  and  potassium 
carbonate,  but  that  it  is  produced  with  greater  certainty  by  dis- 
solving potassium  carbonate  and  telluric  acid  in  a small  amount 
of  hot  water  in  such  proportions  that  there  shall  be  two  molecules 
of  telluric  acid  present  to  one  of  potassium  carbonate.  When 
such!  a solution  is  cooled  the  bitellurate  of  potassium  separates 
upon  the  walls  of  the  containing  vessel  in  granular  tufts.  The 
formula  K2O.2TeO3.4H2O  is  assigned  to  the  crystalline  salt. 

Berzelius  states  that  potassium  quadrotellurate  (K20.4Te03) 
can  be  obtained  by  the  same  method  as  the  bitellurate.  In  the 
case  of  the  quadrotellurate,  however,  telluric  .acid  and  potassium 
carbonate  were  dissolved  in  the  proportion  of  four  molecules  of 
the  former  to  one  of  the  latter.  The  formula  K2O.4TeO3.4H2O 

31 


70 


BULLETIN  OF  THE  UNIVEESITY  OF  WISCONSIN. 


is  assigned  to  the  crystalline  salt.  Berzelius  states  that  these 
salts  may  also  be  obtained  by  treating  a solution  of  normal  potas- 
sium tellurate  with  the  requisite  amount  of  an  acid  (nitric  or  sul- 
phuric) to  withdraw  potassium. 

Berzelius  also  describes  a yellow  insoluble  quadrotellurate  of 
potassium  obtained  by  heating  either  K20.2Te03  or  K20.4Te03. 
He  states  that  this  compound  may  also  be  obtained  by  heating  tel- 
luric acid  with  potassium  nitrate  at  a temparature  below  a red 
heat.  The  compound  is  insoluble  in  cold  nitric,  hydrochloric,  and 
sulphuric  acids,  and  potassium  hydrate. 


KHTeO,. 

A tellurate  of  potassium  containing  more  telluric  acid  than 
KHTe04  cannot  be  obtained  by  the  action  of  carbon  dioxide  on 
normal  potassium  tellurate.  When  carbon  dioxide  is  passed  into 
a concentrated  solution  of  potassium  tellurate  a white  precipitate 
of  potassium  bitellurate  (KHTe04)  is  immediately  formed.  The 
precipitate  may  settle  as  a gummy  mass,  but  it  becomes  granular 
when  cooled  to  zero.  The  composition  of  this  precipitate  is  not 
changed  by  prolonged  contact  with  carbon  dioxide.  KHTe04, 
obtained  in  this  manner,  was  repeatedly  covered  with  distilled 
water,  cooled  to  zero,  and  treated  with  carbon  dioxide. 

Analysis  of  the  residue,  after  having  been  dried  with  filter 
paper,  gave  the  following  results : 

0.5428  grams  with  hydrochloric  acid  liberated  chlorine  equiva- 
lent to  0.2415  grams  Te. 

0.9906  grams  gave  0.251 1 grams  KCl. 

1.5444  grams  gave  0.3492  grams  H2O. 

Found  44.49%  Te;  13.30%  K;  and  22.62%  HoO. 

After  deducting  the  water,  some  of  which  was  evidently  held 
mechanically,  the  values  obtained  for  potassium  and  tellurium 
correspond  closely  with  those  required  by  the  formula  K20.2Te03. 
In  another  experiment  18%  of  water  was  obtained  from  a sample 
dried  with  filter  paper,  while  the  same  sample  gave  13.2%  of 
water  after  having  been  dried  over  phosphorus  pentoxide  for  sev- 
eral days.  The  calculated  value  for  water  in  KHTe04.2H20  is 

32 


HUTCHINS OHEMISTllY  OF  THE  TELLURATES. 


71 


i6.8%,  and  for  KHTeO,.i>4HO  (or  K2O.2TeO3.4H2O)  13.9%. 
The  latter  formula  is  the  one  assigned  to  the  bitellurate  by  Berze- 
lius. 

Potassium  bitellurate  may  be  obtained  in  the  form  of  a trans- 
parent crystalline  crust  by  slow  evaporation  of  its  solutions.  It 
is  only  sparingly  soluble  in  water  at  0°  ; but  is  readily  soluble  in 
boiling  water.  It  is  alkaline  to  litmus. 

When  a solution  of  potassium  bicarbonate  is  added  tO'  a con- 
centrated solution  of  normal  potassium  tellurate  a white  precipi- 
tate is  formed.  The  precipitate  appears  flocculent  when  it  is  first 
formed,  but  it  settles  as  a gumm.y  mass.  The  precipitate  is  so 
soluble  in  the  menstruum  present  that  a large  amount  of  potassium 
tellurate  is  required  to  obtain  a small  amount  of  the  solid  ma- 
terial. The  precipitate  is  probably  potassium  bitellurate.  Its 
solubility  in  the  menstruum  may  be  due  to  potassium  carbonate 
formed  by  the  reaction — 

KHCO3  + K^TeO^  = KHTeO^  + K2CO3. 

This  reaction  is  in  harmony  with  the  fact,  mentioned  above, 
that  potassium  bitellurate  does  not  react  with  potassium  carbon- 
ate to  give  carbon  dioxide.  The  ready  formation  of  potassium 
bitellurate  may  be  due  largely  to  the  insolubility  of  the  compound, 
but  from  the  facts  that  have  been  mentioned  it  seems  probable 
that  telluric  acid  is  a weaker  acid  than  carbonic. 


K,0.3Te0,.5H,0. 

When  solutions  made  by  dissolving  potassium  carbonate  and 
telluric  acid  together  in  water  in  the  proportion  of  one  molecule 
of  the  carbonate  to  four  of  the  acid  are  evaporated,  a white  gran- 
ular precipitate  of  K2O.3TeO3.5H2O  is  deposited.  Analysis  of 
the  salt,  after  having  been  dried  for  eleven  days  over  phosphorus 
pentoxide,  gave  the  following  results : 

1.6971  grams  gave  0.2075  grams  H2O. 

1.6971  grams  gave  0.3495  grams  KCl. 

1-3340  grams  gave  (with  HCl)  chlorine  equivalent  to  0.7207 
grams  Te. 


72 


BULLETIN  OF  THE  UNIVERSITY  OF  WISCONSIN. 


Calculated  for  K2O.3TeO3.5H2O 

Found 

H20 

12.65 

12.22 

K 

II.OI 

11.06 

Te 

53-83 

54.02 

The  salt  reacts  alkaline  to  litmus.  Like  the  bitellurate  it  is 
much  more  soluble  in  hot  water  than  in  cold. 

This  acid  salt  may  be  obtained  by  dissolving  potassium  carbon- 
ate and  telluric  acid  together  in  the  ratio  of  one  molecule  of  potas- 
sium carbonate  tO'  three  of  telluric  acid. 

MERCUROUS  TELLURATES. 

Berzelius describes  normal  mercurous  tellurate  (Hg2Te04) 
as  a yellow-brown  precipitate  obtained  by  adding  powdered  mer- 
curous nitrate  to  a solution  of  potassium  tellurate.  When  a solu- 
tion of  mercurous  nitrate  is  used  instead  of  the  powdered  salt, 
the  yellow-brown  precipitate  gradually  changes  to  a pale  yellow 
color.  Berzelius  states  that  this  change  in  color  is  probably  due 
to  the  formation  of  the  bitellurate  of  mercury  (HgHTe04) 
brought  about  by  the  free  acid  in  the  solution  of  the  mercury  salt. 

Oppenheim^^  states  that  when  a concentrated  solution  of 
mercurous  nitrate  is  treated  with  telluric  acid  a white  cheesy  pre- 
cipitate forms  which  becomes  yellow  in  the  air.  He  considers 
this  precipitate  a double  salt  of  mercurous  nitrate  and  tellurate 
analogous  to  the  double  compounds  of  silver  and  lead  that  he 
describes. 

The  precipitates  obtained  by  double  decomposition  of  solutions 
of  mercurous  nitrate  and  a soluble  tellurate  or  telluric  acid  may 
vary  in  composition  from  HHgTe04  to  3Hg0.2Te03. 

However,  any  precipitate  of  mercurous  tellurate  may  be 
changed  to  crystals  of  HHgTe04.3H20  by  treatment  with  a cold 
concentrated  solution  of  telluric  acid.  This  acid  tellurate  is  not 
only  the  only  crystalline  mercurous  tellurate  that  has  been'  pre- 
pared, but  it  is  the  only  mercurous  tellurate  of  definite  composi- 
tion that  has  been  obtained  during  the  progress  of  this  work. 


«Pogg.  Ann.,  Vol.  32,  p.  577. 

Jr.  f.  pr.  Chem.,  Vol.  71,  p.  266. 


34 


HUTCHINS CITEMlSTilY  OF  THE  TELLURATES. 


I 


i o 


Mercurous  tellurate  is  a striking  example  of  a salt  of  a weak 
acid  and  a weak  base.  Slight  variations  of  the  conditions  under 
which  a salt  is  placed  may  bring  about  a change  in  its  composi- 
tion, causing  it  to  become  either  more  acid  or  more  basic.  This, 
together  with  the  fact  that  the  salt  is  readily  decomposed  giving 
free  mercury  and  mercuric  tellurate,  renders  the  preparation  of 
the  tellurates  of  monovalent  mercury  somewhat  difficult. 


HHgTe0,.3H,0. 

This  crystalline  salt  may  be  obtained  either  by  double  decom- 
position of  solutions  of  mercurous  nitrate  and  telluric  acid  or  by 
the  action  of  telluric  acid  upon  mercurous  oxide. 

When  mercurous  oxide  is  treated  with  a cold  concentrated  solu- 
tion of  telluric  acid  it  is  slowly  attacked  and  colorless  crystals  of 
HHgTe04.3H20  are  formed.  As  the  resulting  crystals  are 
mixed  with  the  unattacked  oxide,  this  is  not  a suitable  method 
for  the  preparation  of  the  salt  in  pure  condition. 

Large  amounts  of  the  salt  may  be  conveniently  prepared  by 
treating  a solution  of  mercurous  nitrate  with  telluric  acid  and 
subsequent  change  of  the  precipitate  to  the  crystalline  salt. 
When  a solution  of  telluric  acid  is  added  to  a solution  of  mercur- 
ous nitrate  a bright  yellow  precipitate  is  formed.  If  a large  ex- 
cess of  telluric  acid  is  added,  the  yellow  precipitate  becomes 
nearly  white  and  slowly  changes  to  colorless  crystals  of 
HHgTe04.3H20.  The  presence  of  considerable  free  nitric  acid 
in  the  solution  aids  greatly  in  the  formation  of  crystals  of  the 
salt.  Under  these  conditions  crystals  frequently  form;  on  the 
walls  of  the  containing  vessel. 

Analysis  of  the  colorless  crystals  gave  the  following  results : 

1.9265  grams  gave  0.2717  grams  H2O. 

0.8459  grams  gave  0.4358  grams  HgS. 

1.2416  grams  with  hydrochloric  acid  liberated  chlorine  equiva- 
lent to  0.3501  grams  Te. 


Calculated  for  HHgTeO^.sH^O 

Found 

Hg 

44.78 

44.40 

Te 

28.57 

28.21 

H2O 

I4.II 

14.11 

35 


74 


BULLETIN  OF  THE  UNIA'ERSITY  OF  WISCONSIN. 


For  the  estimation  of  the  mercury,  all  of  which  was  present 
in  the  mercurous  condition,  the  salt  was  treated  with  dilute  hy- 
drochloric acid  and  the  precipitated  mercurous  chloride  oxidized 
with  bromine  water.  The  mercury  was  precipitated  with  hydro- 
gen sulphide. 

Tellurium  was  estimated  by  decomposing  the  salt  with  hydro- 
chloric acid,  absorbing  the  liberated  chlorine  in  a solution  of 
potassium  iodide  and  titrating  the  free  iodine  with  sodium  thio- 
sulphate. The  chlorine  absorbed  by  the  solution  of  potassium 
iodide  represents  only  a portion  of  the  chlorine  liberated  by  the 
oxidation  of  the  hydrochloric  acid.  When  the  tellurate  is  treated 
with  hydrochloric  acid  in  the  cold,  mercurous  choride  is  precipi- 
tated. Upon  heating  the  solution  the  chlorine  formed  by  the 
oxidation  of  the  hydrochloric  acid  converts  the  mercurous  chlor- 
ide into  mercuric  chloride.  The  amount  of  chlorine  thus  reduced 
was  calculated  and  added  to  the  amount  absorbed  by  the  iodide 
solution  in  order  to  determine  the  amount  of  tellurium  in  the  salt. 
The  reactions  involved  may  be  represented  as  follows : 
2HHgTe04  + 2HCI  = Hg^Cl^ 

2U^TqO,  -f  12HCI  =:  2TeCl4  + 8H2O+2CI2. 

Hg2Cl2  + Cl2  = 2HgCl2. 

One-half  of  the  chlorine  liberated  during  the  reaction  is  re- 
duced by  the  mercurous  chloride  in  the  solution. 

HgHTe04.3H20  crystallizes  in  the  triclinic  system.  It  is  in- 
soluble in  water  but  soluble  in  dilute  acetic  or  nitric  acid.  Am- 
monium hydroxide  reacts  with  the  salt  tO'  form  a black  precipitate. 
Sunlight  blackens  the  salt,  but  it  is  stable  in  the  air  if  protected 
from  the  light.  Crystals  of  the  salt  have  been  preserved  in 
closed  tubes  for  a year  without  losing  their  transparency  or 
changing  in  any  respect.  With  boiling  water  the  crystals  quickly 
decompose,  telluric  acid  and  a yellow  amorphous  tellurate  being 
formed.  An  excess  of  cold  concentrated  mercurous  nitrate  solu- 
tion brings  about  the  same  result.  If  the  yellow  basic  precipitate 
is  treated  with  an  excess  of  telluric  acid  it  may  be  changed  to- 
crystals  of  HHgTe04.3H20.  When  strongly  compressed  be- 
tween steel  rolls  the  salt  detonates  and  assumes  a brown  color. 
The  amorphous  mercurous  tellurates  possess  the  same  property. 
It  has  been  observed  that  when  a glass  rod  is  drawn  over  the  wet 

36 


i 


HUTCHINS CHEMISTEY  OF  THE  TELLUEATES.  75 

precipitate  obtained  by  bringing  together  solutions  of  mercurous 
nitrate  and  potassium  tellurate,  it  changes  from  a yellow  to  a 
brown  color.  The  same  change  may  be  brought  about  by  press- 
ing the  precipitate  between  layers  of  filter  paper. 


37 


7G 


bulletin  of  the  UNIVEKSITY  OF  WISCONSIN. 


HHgTeO^.sH^O. 

Triclinic.  Axes — a :b  :c=  1.044  • i • i-055 


106°  03 

y= 

1103'^ 

' 59'- 

ca, 

001 

: 100  = 

: 109° 

3S'. 

ab, 

100 

:oio  = 

108° 

00'. 

cb, 

001 

:oio  = 

:I05° 

30'. 

cs, 

001 

:oi  I = 

:I4I° 

45'- 

cx, 

001 

:Tii  = 

: 127° 

V * 

0 

sx, 

on 

:Tii  = 

:I4I° 

30'. 

c'r, 

ooT  : 

TiT= 

131° 

32'. 

as, 

100 

:oii  = 

114° 

05'- 

bx. 

010 

:Tii  = 

1130° 

15'- 

c'h, 

ooT  : 

:oiT= 

126° 

08'. 

hs, 

oiT 

:oii  = 

: 92° 

23'. 

ub, 

223  : 

:oio= 

133° 

25'- 

uc', 

223  ; 

ooT= 

103° 

30'. 

rb, 

Tii  : 

010= 

106° 

00'. 

xa', 

Tii  : 

Too= 

104° 

25'- 

1 


HUTCHINS OHEMISTEY  OF  THE  TELLURATES. 


7T 


Other  Mercurous  Tcllurates. 

All  attempts  to  prepare  normal  mercurous  tellurate  in  the  crys- 
talline form  have  been  unsuccessful.  Indeed,  it  has  not  been 
found  possible  to  obtain  an  amorphous  tellurate  of  mercury  hav- 
ing exactly  the  composition  Hg2Te04.  The  salt  is  likely  to  be 
contaminated  by  either  a basic  or  an  acid  tellurate  of  mercury, 
depending  upon  the  conditions  in  the  solution.  The  preparation 
of  the  salt  is  still  further  complicated  by  the  fact  that  the  precipi- 
tate obtained  by  double  decomposition  of  mercurous  nitrate  and 
potassium  tellurate  contains  nitric  acid  which  it  is  difficult  to 
remove  by  washing.  Moreover,  long  continued  treatment  with 
water  decomposes  the  precipitate. 

When  a large  excess  of  a cold  concentrated  solution  of  potas- 
sium tellurate  is  added  to  a concentrated  solution  of  mercurous 
nitrate  the  yellow  precipitate,  which  is  first  formed,  quickly 
changes  to  a brown.  The  precipitate  is  slimy  and  difficult  to 
wash.  It  approximates  Hg2Te04  in  composition. 

Impure  3Hg20.2Te03  may  be  obtained  as  a yellow  amorphous 
precipitate  by  treating  a hot  dilute  solution  of  potassium  tellurate 
with  an  excess  of  mercurous  nitrate  solution  and  washing  the  pre- 
cipitate with  hot  water.  It  is  hardly  possible  to  obtain  the  pre- 
cipitate free  from  nitric  acid. 

A mercurous  tellurate  analogous  or  silver  orthotellurate 
(AggTeOg)  has  not  been  prepared.  When  the  bulky  yellow  pre- 
cipitate, obtained  by  treating  a hot  dilute  solution  of  potassium 
tellurate  with  an  excess  of  mercurous  nitrate  solution  is  washed 
with  hot  water  and  boiled  for  some  time  in  water  it  is  decom- 
posed giving  free  mercury.  The  precipitate  may  be  boiled  for 
several  hours  without  changing  in  appearance,  but  at  length  it 
settles  as  a heavy  brown  granular  powder  containing  free  mer- 
cury. This  change  may  be  hastened  by  rubbing  some  of  the  pre- 
cipitate against  the  side  of  the  beaker  with  a glass  rod. 

When  the  brown  powder  is  heated  with  hydrochloric  acid  ele- 
mentary tellurium  is  precipitated. 


39 


78 


BULLETIN  OF  THE  UNIVERSITY  OF  WISCONSIN. 


MERCURIC  TELLURATES. 

Berzelius  d'escribes  mercuric,  tellurate  (HgTe04)  as  a white 
flocculent  precipitate. 

Divalent  mercury  forms  two  salts  with  telluric  acid,  mer- 
curic orthotellurate  (HggTeOe)  normal  mercuric  tellurate 
(HgTe04),  both  of  which  have  been  prepared  in  crystalline  form. 
HgTe04  may  be  obtained  as  a white  flocculent  precipitate  as  de- 
scribed by  Berzelius.  Under  favorable  conditions  it  crystallizes 
with  two  molecules  of  water,  giving  HgTe04.2H20.  The  in- 
termediate compound  3Hg0.2Te03  has  not  been  isolated,  al- 
though there  are  indications  that  it  may  exist.  All  precipitates 
of  mercuric  tellurate,  .whether  crystalline  or  amorphous,  show  a 
strong  tendency  in  the  presence  of  water  to  change  to  mercuric 
orthotellurate.  This  property  of  mercuric  tellurate  renders  it  dif- 
ficult to  prepare  HgTe04  in  pure  condition. 


M erciiric  Orthotellurate — Hg^TeO^. 

When  an  excess  of  mercuric  nitrate  is  added  to  a hot  dilute 
solution  of  potassium  tellurate,  a heavy  yellow  granular  precipi- 
tate of  mercuric  orthotellurate  is  formed.  In  order  to  obtain 
this  salt  in  crystalline  form,  a somewhat  different  method  of  pro- 
cedure must  be  employed. 

If  a cold  concentrated  solution  of  mercuric  nitrate  is  added  to 
a concentrated  solution  of  potassium  tellurate  made  acid  with 
nitric  acid,  a white  flocculent  precipitate  of  HgTe04  is  formed. 
At  room  temperatures  a portion  of  the  precipitate  usually  as- 
sumes a yellow  color  immediately,  probably  due  to  the  formation 
of  a basic  salt.  If  the  acid  solution  containing  this  white  or 
yellow  flocculent  precipitate  is  allowed  to  stand  undisturbed  for 
several  days  the  precipitate  is  largely  converted  into  transparent 
amber  colored  crystals  of  HggTeOe.  Frequently  the  walls  of 
the  containing  vessel  are  covered  with  crystals  that  have  separ- 
ated from  the  acid  solution.  In  many  cases  the  amber  colored 
crystals  form  in  contact  with  the  white  amorphous  precipitate,  but 

40 


4 


HUTCHINS CHEMISTRY  OF  THE  TELLURATES/  '79 

in  all  cases  the  crystals  are  entirely  distinct  from  the  precipitate. 

HggTeOe  crystallizes  in  the  isometric  system,  the  predominat- 
ing form  being  the  dodecahedron.  Combinations  of  the  cube  and 
dodecahedron  are  frequent,  while  many  crystals  show  a broad 
octahedral  face  on  which  the  crystal  has  grown.  Single  crystals 
have  been  obtained  which  are  over  an  eighth  of  an  inch  in  diam- 
eter. The  crystals  contain  a little  nitric  acid  and  about  o.6% 
of  water.  This  is  considerably  less  than  one-half  of  a molecule 
of  water  to  one  of  HggTeOs. 

The  crystals  are  insoluble  in  water  and  unchanged  by  boiling 
with  water.  They  are  soluble  in  nitric  acid  but  more  readily 
soluble  in  hydrochloric  acid.  Hot  potassium  hydrate  precipitates 
mercuric  oxide.  The  powdered  salt  does  not  change  in  compo- 
sition when  kept  in  contact  with  a cold  saturated  solution  of 
telluric  acid  for  '48  hours. 

When  heated  the  salt  assumes  a red  color,  but  it  regains  its 
original  color  upon  cooling.  The  crystals  are  not  altered  by 
heating  to  140°. 

Analysis  of  the  salt,  after  having  been  dried  over  phosphorus 
pentoxide  for  several  days,  gave  the  following  results : 

4.6696  grams  gave  0.0279  grams  H2O. 

1.7853  grams  gave  1.4940  grams  HgS. 

1.0894  grams  with  hydrochloric  acid  liberated  chlorine  equiva- 
lent to  0.1727  grams  Te. 

Found  72.13%  Hg  15-85%  Te  0.59%  H2O 

The  value  obtained  for  tellurium  is  about  0.5%  too  high.  This 
discrepancy  is  due  to  the  presence  of  a little  nitric  acid  in  the 
crystals. 

The  corresponding  sulphate  of  mercury,  Hg^SOg,  exists  as  tur- 
peth  mineral.  This  compound  may  be  prepared  as  a heavy  yel- 
low powder  or  as  yellow  crystals  by  treating  the  normal  sulphate 
with  water.  In  color  the  sulphate  is  very  similar  to  the  tellurate, 
but  it  does  not  crystallize  in  the  isometric  system. 

'Amorphous  Hg^TeO^, 

When  concentrated  solutions  of  mercuric  nitrate  and  potassium 
tellurate  or  telluric  acid  are  brought  together  at  zero  a white 

41 


{ 


80 


BULLETIN  OF  THE  UNIVERSITY  OF  WISCONSIN. 


flocculent  precipitate  of  mercuric  tellurate  is  formed.  The  pre- 
cipitate is  quickly  decomposed  by  water  at  room  temperature  into 
telluric  acid  and  basic  mercuric  tellurate.  This  decomposition 
goes  on  even  when  the  precipitate  is  washed  with  water  at  zero; 
consequently  it  is  impossible  to  obtain  a pure  white  product  in 
suitable  condition  for  analysis.  The  precipitate  remains  white 
indefinitely  in  a saturated  solution  of  telluric  acid  at  room  tem- 
perature. 

Analysis  of  the  light  yellow  powder  obtained  by  washing  the 
white  precipitate  with  water  at  o°  showed  that  it  approximates 
HgTe04  in  composition. 

Crystalline  HgTeO^^.^H^O. 

If  the  white  precipitate  of  mercuric  tellurate  formed  by  mixing 
concentrated  solutions  of  mercuric  nitrate  and  potassium  tellurate 
containing  free  nitric  acid,  is  allowed  to  remain  undisturbed  in  the 
mother  liquor  for  several  days  at  room  temperature,  nearly  all  of 
it  is  converted  into  crystalline  HggTeOe.  In  some  cases  colorless 
crystals  of  HgTe04.2H20  were  obtained  mixed  with  the  amber 
colored  crystals  of  mercuric  orthotellurate. 

Analysis  of  the  colorless  crystals  gave  the  following  results : 

0.2413  grams  gave  0.0202  grams  H2O ; 

0.2413  grams  gave  0.0887  grams  Te02 ; 

Calculated  for  HgTe04.2H20'  Found 

Te  29.84  29.39 

H,0  8.41  8.37 

HgTe04.2H20  appears  as  transparent  crystals  belonging  to  the 

orthorhombic  system.  It  is  slowly  decomposed  by  cold  water. 
Boiling  water  quickly  decomposes  it  into  telluric  acid  and  basic 
mercuric  tellurate. 


Copper  Ortho  tellur  a,fe,  Cu^TeO^. 

When  a solution  of  copper  nitrate  is  treated  with  a solution  of 
potassium  tellurate  a green  flocculent  precipitate  of  copper  tel- 
lurate is  formed.  Berzelius  describes  this  compound  as  normal 
copper  tellurate  (CuTe04).  If  the  precipitate  is  boiled  for  some 

42 


HUTCHIInS CHEMISTEY  OF  THE  TELLURATES.  81 

hours  with  water  it  is  converted  into  copper  orthotellurate,  which 
settles  as  a heavy  green  powder.  It  is  insoluble  in  water  but 
soluble  in  ammonia,  potassium  cyanide,  and  acetic,  hydrochloric, 
and  nitric  acids. 

Analysis  of  the  green  powder  after  having  been  dried  over 
phosphorus  pentoxide  for  a week  gave  the  following  result : 

0.6675  grams  heated  with  hydrochloric  acid  liberated  chlorine 
equivalent  to  0.2069  grams  tellurium. 

CugTeOe  requires  30.79%  tellurium;  found  31.00%. 

Numerous  attempts  were  made  to  prepare  copper  tellurate  in 
crystalline  form,  but  none  were  successful.  Copper  hydrate  was 
treated  with  an  excess  of  telluric  acid ; the  resulting  tellurate  of 
copper  was  allowed  to  remain  in  the  solution  for  several  weeks. 
Likewise  the  precipitate  obtained  by  bringing  together  solutions 
of  potassium  tellurate  and  copper  nitrate  was  allowed  to  remain 
in  mother  liquors  of  varying  composition  for  weeks.  In  no'  case 
were  any  crystals  of  copper  tellurate  formed. 

Zinc  Orthotellurate,  Zn^TeO^. 

The  white  flocculent  precipitate  obtained  by  bringing  together 
solutions  of  zinc  nitrate  and  potassium  tellurate  may  be  changed 
into  a heavy  granular  precipitate  of  ZugTeOo  by  long  continued 
treatment  with  hot  water. 

Analysis  of  the  powder  prepared  in  this  manner  and  dried  over 
phosphorus  pentoxide  for  a week  gave  the  following  result; 
0.6331  grams  with  hydrochloric  acid  liberated  chlorine  equivalent 
to  0.1940  grams  tellurium.  Calculated  for  ZusTeOg,  30.40% 
tellurium-;  found  30.64%. 

ZngTeOg  is  insoluble  in  water  but  soluble  in  acetic,  nitric,  hy- 
drochloric, and  sulphuric  acids. 

Gutbier^®  prepared  zinc  tellurate  in  the  form  of  a white  insol- 
uble precipitate  by  bringing  together  solutions  of  potassium  tellu- 
rate and  zinc  chloride.  He  does  not  give  the  composition  of  the 
tellurate  of  zinc  that  he  obtained  in  this  manner. 


^ Zeit.  f.  anorg.  Chem.,  Vol.  31,  p,  349. 

43 


82 


bulletin  of  the  university  or  Wisconsin. 


Attempts  were  made  to  prepare  zinc  tellurate  in  crystalline 
form  by  allowing  the  amorphous  salt  to  remain  in  contact  with 
mother  liquors  of  various  compositions.  The  results  were  all 
negative. 


GOLD  TELLURATU. 

No  tellurate  of  gold  has  been  prepared.  From  the  experiments 
that  have  been  made  in  an  attempt  to  prepare  this  salt  it  seems 
probable  that  water  immediately  decomposes  the  salt  if  it  is 
formed  in  aqueous  solutions. 

When  a solution  of  gold  chloride  is  added  to  normal  silver  tel- 
lurate suspended  in  water,  silver  chloride  and  gold  oxide  are  pre- 
cipitated. 

When  solutions  of  normal  potassium  tellurate  and  gold  chloride 
are  brought  together  and  the  resulting  solution  allowed  to  evap- 
orate over  sulphuric  acid,  yellow  crystals  of  potassium  chlor- 
aurate  are  formed.  No  tellurate  of  gold  separates  from  the  solu- 
tion. 

The  fact  that  gold  chloride  takes  up  potassium  from  potassium 
tellurate  to  form  potassium  chlor-aurate  is  a good  illustration  of 
the  weak  affinity  that  telluric  acid  exerts  towards  bases. 


separation  of  gold  from  tellurium. 

While  working  with  gold  and  telluric  acid  it  became  necessary 
to  separate  gold  from  tellurium.  It  was  found  that  nitrous  acid 
would  not  reduce  either  telluric  or  tellurous  acids.  Nitrous  acid 
completely  reduces  gold  from  its  solutions. Gold  may  be  sepa- 
rated from  tellurium  in  dilute  acid  solution  by  the  addition  of 
potassium'  nitrate.  After  filtering  out  the  precipitated  gold,  tellu- 
rium may  be  determined  in  the  filtrate  by  precipitation  with  sul- 
phur dioxide. 


Fischer,  Pogg.  Ann.,  Vol.  17,  p.  480. 


HUTCHINS CHEMISTEY  OF  THE  TELLURATES. 


83 


SUMMARY. 

1.  -Telluric  acid  forms  ortho-tellurates  of  the  type  M'gTeOg, 
with  a number  of  the  metals.  AggTeOg,  HggTeOg,  Zn.TeOg, 
and  CugTeOg  have  been  prepared.  The  normal  tellurates  (e.  g. 
Ag2Te04)  of  these  metals  are  changed  tO'  the  ortho  compounds 
by  treatment  with  water. 

2.  The  crystalline  normal  tellurates  that  have  been  prepared, 
viz. : Ag2Te04.2H20,  Rb^TeO^-sH^O,  HgTe04.2H20,  Cs2Te04. 
3H2O,  Na2Te04.2H20,  and  K2Te04.2H  may  be  considered 
acid  salts  of  orthotelluric  acid  of  the  types  M'2H4TeO'g  and 
M'2H4Te0e.H20. 

3.  Silver  forms  two  crystalline  tellurates — Ag2O.TeO3.2H2O 
and  3Ag2O.2TeO3.3H2O.  The  former  salt  is  yellow  and  crystal- 
lizes^ in  the  orthorhombic  system ; the  latter  is  red  and  crystallizes 
in  the  monoclinic  system.  Both  of  these  salts  may  be  considered 
acid  salts  of  orthotelluric  acid.  No  selenate  or  sulphate  of  silver 
is  known  that  is  isomorphous  with  either  of  these  salts. 

4.  HgHTe04.3H20  crystallizes  in  the  triclinic  system.  It  is 
the  only  crystalline  mercurous  tellurate  that  has  been  prepared. 

5.  Two  crystalline  salts  of  divalent  mercury  have  been  pre- 
pared, viz. : Hg3Te06  and  HgTe04.2H20.  The  former  appears 
in  the  form  of  amber  colored  crystals  belonging  to  the  isometric 
system ; the  latter  is  white  and  crystallizes  in  the  orthorhombic 
system. 

6.  The  crystalline  compound  described  by  Oppenheim  as  a 
double  salt  of  silver  nitrate  and  tellurate  is  normal  silver  tellurate 
{Ag,Tt0,.2H,0). 

7.  It  has  not  been  found  possible  to  prepare  the  acid  tellurates 
of  silver  described  by  Berzelius. 

8.  Potassium  tellurate  may  be  prepared  in  crystalline  form  by 
slow  evapouation  of  its  solution,  provided  care  is  exercised  to  pre- 
vent supersaturation  of  the  solution. 

Potassium  forms  a second  tellurate — K^TeO^.  5H2O. 

45 


84r  BULLETIN  OF  THE  UNIVERSITY  OF  WISCONSIN. 

9.  Telluric  acid  does  not  completely  replace  the  carbonic  acid 
in  an  equivalent  quantity  of  potassium  carbonate.  Crystalline 
normal  potassium  tellurate  cannot  be  obtained  from  potassium 
carbonate  and  telluric  acid  as  described  by  Berzelius. 

10.  Although  telluric  acid  is  a weak  acid,  hot  concentrated 
solutions  of  it  attack  mercury,  silver,  lead,  tin,  arsenic,  antimony, 
bismuth,  nickel,  zinc,  aluminum,  and  cadmium. 

11.  There  are  no  well  authenticated  cases  of  isomorphism  be- 
tween sulphates  and  tellurates  or  between  selenates  and  tellurates. 


This  work  was  undertaken  at  the  suggestion  of  Professor 
Lenher  and  carried  out  under  his  guidance.  I wish  to  take  this 
opportunity  of  expressing  my  thanks  for  the  inspiration  that  I 
have  received  from  him  in  the  laboratory  as  well  as  in  the  class- 


room. 


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